November 2016.

We investigate algorithms
that learn to run faster: given a series of instances of a computational
problem, these algorithms use characteristics of past instances to sharpen
their performance for new instances. We have found such *self-improving*
algorithms for sorting a list of numbers, and for some problems on planar
point sets: computing Delaunay triangulations, coordinate-wise maxima, and
convex hulls. Under the assumption that input instances are random variables,
each algorithm begins with a training phase during which it adjusts itself to
the distribution of the input instances; this is followed by a stationary
regime in which the algorithm runs in its optimized version. In this setting,
an algorithm must take expected time proportional to the input size `n`, and
to the entropy `E` of the output: the input must be touched, and the output
must be communicated. Our algorithms achieve `O(n+E)` expected running times
in the stationary regime for all the problems mentioned except convex hulls,
where `O(n\log\log n + E)` expected time is needed.

July 2016.

From the excruciatingly difficult to the
achingly elegant, Bernard Chazelle's work on algorithms, especially geometric
or natural ones, has been profoundly influential. I'll sketch a few examples
that have been inspiring to me, including 1-dimensional range queries,
low-stabbing spanning trees, high-order Voronoi diagram construction,
deterministic constructions, and the s-energy of a system.

In the subspace approximation problem, we
seek a $k$-dimensional subspace $F$ of $\R^d$ that minimizes the sum of
$p$-th powers of Euclidean distances to a given set of $n$ points $a_1,
\ldots, a_n \in \R^d$, for $p \geq 1$. More generally than minimizing $\sum_i
\dist(a_i,F)^p$, we may wish to minimize $\sum_i M(\dist(a_i,F))$ for some
loss function $M()$, for example, $M$-Estimators, which include the Huber and
Tukey loss functions. Such subspaces provide alternatives to the singular
value decomposition (SVD), which is the $p=2$ case, finding such an $F$ that
minimizes the sum of squares of distances. For $p \in [1,2)$, and for typical
$M$-Estimators, the minimizing $F$ gives a solution that is more robust to
outliers than that provided by the SVD. We give several algorithmic results
for these robust subspace approximation problems. We state our results as
follows, thinking of the $n$ points as forming an $n \times d$ matrix $A$,
and letting $\nnz{A}$ denote the number of non-zero entries of $A$. Our
results hold for $p\in [1,2)$. We use $\poly(n)$ to denote $n^{O(1)}$ as
$n\rightarrow\infty$.

- For minimizing $\sum_i \dist(a_i,F)^p$, we give an algorithm running in \[ O(\nnz{A} + (n+d)\poly(k/\eps) + \exp(\poly(k/\eps))) \] time which outputs a $k$-dimensional subspace $F$ whose cost is at most a $(1+\eps)$-factor larger than the optimum.
- We show that the problem of minimizing $\sum_i \dist(a_i, F)^p$ is NP-hard, even to output a $(1+1/\poly(d))$-approximation. This extends work of Deshpande et al. (SODA, 2011) which could only show NP-hardness or UGC-hardness for $p > 2$; their proofs critically rely on $p > 2$. Our work resolves an open question of [Kannan Vempala, NOW, 2009]. Thus, there cannot be an algorithm running in time polynomial in $k$ and $1/\eps$ unless P = NP. Together with prior work, this implies that the problem is NP-hard for all $p \neq 2$.
- For loss functions for a wide class of $M$-Estimators, we give a problem-size reduction: for a parameter $K=(\log n)^{O(\log k)}$, our reduction takes \[ O(\nnz{A}\log n + (n+d)\poly(K/\eps)) \] time to reduce the problem to a constrained version involving matrices whose dimensions are $\poly(K\eps^{-1}\log n)$. We also give bicriteria solutions.
- Our techniques lead to the first $O(\nnz{A} + \poly(d/\eps))$ time algorithms for $(1+\eps)$-approximate regression for a wide class of convex $M$-Estimators. This improves prior results, which were $(1+\eps)$-approximation for Huber regression only, and $O(1)$-approximation for a general class of $M$-Estimators.

Tutorial for Gene Golub Summer School, July 2015.

The outline:

We give algorithms for the
*$M$-estimators* $\min_x \norm{Ax-b}_G$, where $A\in\R^{n\times d} $
and $b\in\R^n$, and $\norm{y}_G$ for $y\in\R^n$ is specified by a cost
function $G:\R\mapsto \R^{\geq 0}$, with $\norm{y}_G \equiv \sum_i G(y_i)$.
The $M$-estimators generalize $\ell_p$ regression, for which $G(x)=|x|^p$. We
first show that the *Huber* measure can be computed up to relative
error $ \epsilon$ in $O(\nnz{A}\log n+ \poly(d (\log n)/\eps))$ time, where
$\nnz{A}$ denotes the number of non-zero entries of the matrix $A$. Huber is
arguably the most widely used $M$-estimator, enjoying the robustness
properties of $\ell_1$ as well as the smoothness properties of $\ell_2$. We
next develop algorithms for general $M$-estimators. We analyze the
*$M$-sketch*, which is a variation of a sketch introduced by Verbin
and Zhang in the context of estimating the earthmover distance. We show that
the $M$-sketch can be used much more generally for sketching any $M$-
estimator provided $G$ has growth that is at least linear and at most
quadratic. Using the $M$-sketch we solve the $M$-estimation problem in
$O(\nnz{A} + \poly(d \log n))$ time for any such $G$ that is convex, making a
single pass over the matrix and finding a solution whose residual error is
within a constant factor of optimal, with high probability.

We design a new distribution over
$\poly(r \eps^{-1}) \times n$ matrices S so that for any fixed $n\times d$
matrix A of rank r, with probability at least 9/10, $\norm{SAx}_2 = (1 \pm
\eps)\norm{Ax}_2$ simultaneously for all $x\in\R^d$. Such a matrix $S$ is
called a *subspace embedding* Furthermore, SA can be computed in
$O(\nnz{A})+ \poly(d \eps^{-1})$ time, where $\nnz{A}$ is the number of
non-zero entries of A. This improves over all previous subspace embeddings,
which required at least $\Omega(nd\log d)$ time to achieve this property. We
call our matrices $S$ *sparse embedding matrices.* Using our sparse
embedding matrices, we obtain the fastest known algorithms for
$(1+\eps)$-approximation for overconstrained least-squares regression,
low-rank approximation, approximating all leverage scores, and
$\ell_p$-regression. The leading order term in the time complexity of our
algorithms is $O(\nnz{A})$ or $O(\nnz{A}\log n)$. We optimize the low-order
$\poly(d/\eps)$ terms in our running times (or for rank-$k$ approximation,
the $n*\poly(k/\eps)$ term), and show various tradeoffs. For instance, we
also use our methods to design new preconditioners that improve the
dependence on $\eps$ in least squares regression to $\log 1/\eps$. Finally,
we provide preliminary experimental results which suggest that our algorithms
are competitive in practice.

We provide fast algorithms for overconstrained
$\ell_p$ regression and related problems: for an $n\times d$ input matrix $A$
and vector $b\in\R^n$, in $O(nd\log n)$ time we reduce the problem
$\min_{x\in\R^d} \norm{Ax-b}_p$ to the same problem with input matrix $\tilde
A$ of dimension $s \times d$ and corresponding $\tilde b$ of dimension
$s\times 1$. Here, $\tilde A$ and $\tilde b$ are a *coreset* for the
problem, consisting of sampled and rescaled rows of $A$ and $b$; and $s$ is
independent of $n$ and polynomial in $d$. Our results improve on the best
previous algorithms when $n\gg d$, for all $p\in [1,\infty)$ except $p=2$; in
particular, they improve the $O(nd^{1.376+})$ running time of Sohler and
Woodruff (STOC, 2011) for $p=1$, that uses asymptotically fast matrix
multiplication, and the $O(nd^5\log n)$ time of Dasgupta *et al.*
(SICOMP, 2009) for general $p$, that uses ellipsoidal rounding. We also
provide a suite of improved results for finding well-conditioned bases via
ellipsoidal rounding, illustrating tradeoffs between running time and
conditioning quality, including a one-pass conditioning algorithm for general
$\ell_p$ problems. To complement this theory, we provide a detdailed
empirical evaluation of implementations of our algorithms for $p=1$,
comparing them with several related algorithms. Among other things, our
empirical results clearly show that, in the asymptotic regime, the theory is
a very good guide to the practical performance of these algorithms. Our
algorithms use our faster constructions of well-conditioned bases for
$\ell_p$ spaces and, for $p=1$, a fast subspace embedding of independent
interest that we call the Fast Cauchy Transform: a matrix $\Pi: \R^n\mapsto
\R^{O(d\log d)}$, found obliviously to $A$, that approximately preserves the
$\ell_1$ norms: that is, $\norm{Ax}_1 \approx \norm{\Pi Ax}_1$, for all $x$,
with distortion $O(d^{2+\eta} \log d)$, for an arbitrarily small constant
$\eta>0$; and, moreover, $\Pi A$ can be computed in $O(nd\log d)$ time. The
techniques underlying our Fast Cauchy Transform include fast
Johnson-Lindenstrauss transforms, low-coherence matrices, and rescaling by
Cauchy random variables.

Finding the coordinate-wise maxima and the
convex hull of a planar point set are probably the most classic problems in
computational geometry. We consider these problems in the *self-improving
setting*. Here, we have $n$ distributions $\cD_1, \ldots, \cD_n$ of
planar points. An input point set $(p_1, \ldots, p_n)$ is generated by taking
an independent sample $p_i$ from each $\cD_i$, so the input is distributed
according to the product $\cD = \prod_i \cD_i$. A *self-improving
algorithm* repeatedly gets inputs from the distribution $\cD$ (which is
*a priori* unknown), and it tries to optimize its running time for
$\cD$. The algorithm uses the first few inputs to learn salient features of
the distribution $\cD$, before it becomes fine-tuned to $\cD$. Let
$\OPTMAX_\cD$ (resp. $\OPTCH_\cD$) be the expected depth of an
*optimal* linear comparison tree computing the maxima (resp. convex
hull) for $\cD$. Our maxima algorithm eventually achieves expected running
time $O(\OPTMAX_\cD + n)$. Furthermore, we give a self-improving algorithm
for convex hulls with expected running time $O(\OPTCH_\cD + n\log\log n)$.
Our results require new tools for understanding linear comparison trees. In
particular, we convert a general linear comparison tree to a restricted
version that can then be related to the running time of our algorithms.
Another interesting feature is an interleaved search procedure to determine
the likeliest point to be extremal with minimal computation. This allows our
algorithms to be competitive with the optimal algorithm for $\cD$.

Tutorial for kickoff workshop of CG Learning Project, June 2011.

The outline:

- Some geometric
problems, algorithms, analysis
- Approximation algorithms
- First order methods (no second derivatives)
- Analysis using
*on-line convex optimization*formalism- But the
problems and algorithms are
*not*on-line

- But the
problems and algorithms are

- Fast randomized approximate evaluation of dot products
- Alternatives to JL

- Matrix Bernstein and applications
- A primal-dual algorithm
- (Significant) algorithmic results are joint with Elad Hazan and David Woodruff

Tutorial for MADALGO & CTIC Summer School on High-Dimensional Geometric
Computing, Aarhus University, Denmark, August 2011.

The idea of a *metric space*
is among the most basic of geometric concepts, and so appears in a great
variety of applications and algorithms, sometimes in disguise. This is a
light survey of concepts and constructions associated with metric spaces,
including:

- Metric transformations
- `\epsilon`-nets, the greedy algorithm, and applications
- A non-greedy algorithm and a few non-`\epsilon`-nets
- Box dimension and coping with finiteness
- Definitions of dimension that make sense for finite sets
- Estimation of dimension using finite samples
- Filtrations
- Of random subsets, k-medians, `\epsilon`-nets
- Neighbors, generalizing "Delaunay neighbors"
- For witness complexes and NN searching
- Interpolation
- Interpolation of scattered data, Laplacians
- Interpolation in metric spaces
- A curious approach via "witness stealing"

We give sublinear-time approximation
algorithms for some optimization problems arising in machine learning, such
as training linear classifiers and finding minimum enclosing balls. Our
algorithms can be extended to some kernelized versions of these problems,
such as SVDD, hard margin SVM, and $L_2$-SVM, for which sublinear-time
algorithms were not known before. These new algorithms use a combination of a
novel sampling techniques and a new multiplicative update algorithm. We give
lower bounds which show the running times of many of our algorithms to be
nearly best possible in the unit-cost RAM model. We also give implementations
of our algorithms in the semi-streaming setting, obtaining the first low pass
polylogarithmic space *and* sublinear time algorithms achieving
arbitrary approximation factor.

We describe an algorithm for computing planar
convex hulls in the self-improving model: given a sequence $I_1, I_2, \ldots$
of planar $n$-point sets, the upper convex hull $\conv(I)$ of each set $I$ is
desired. We assume that there exists a probability distribution $D$ on
$n$-point sets, such that the inputs $I_j$ are drawn independently according
to $D$. Furthermore, $D$ is such that the individual points are distributed
independently of each other. In other words, the $i$'th point is distributed
according to $D_i$. The $D_i$'s can be arbitrary but are independent of each
other. The distribution $D$ is not known to the algorithm in advance. After a
learning phase of $n^\epsilon$ rounds, the expected time to compute
$\conv(I)$ is $O(n + H(\conv(I)))$. Here, $H(\conv(I))$ is the entropy of the
output, which is a lower bound for the expected running time of *any*
algebraic computation tree that computes the convex hull. (More precisely,
$H(\conv(I))$ is the minimum entropy of any random variable that maps $I$ to
a description of $\conv(I)$ and to a labeling scheme that proves
nonextremality for every point in $I$ not on the hull.) Our algorithm is thus
asymptotically optimal for $D$.

We consider the set multi-cover
problem in geometric settings. Given a set of points $P$ and a collection of
geometric shapes (or sets) $\cal F$, we wish to find a minimum cardinality
subset of $\cal F$ such that each point $p \in P$ is covered by (contained
in) at least $\textrm{d}(p)$ sets. Here $\textrm{d}(p)$ is an integer demand
(requirement) for $p$. When the demands $\textrm{d}(p)=1$ for all $p$, this
is the standard set cover problem. The set cover problem in geometric
settings admits an approximation ratio that is better than that for the
general version. In this paper, we show that similar improvements can be
obtained for the multi-cover problem as well. In particular, we obtain an
$O(\log\ OPT)$ approximation for set systems of bounded VC-dimension, and an
$O(1)$ approximation for covering points by half-spaces in three dimensions
and for some other classes of shapes.

We give near-optimal space bounds in the
streaming model for linear algebra problems that include estimation of matrix
products, linear regression, low-rank approximation, and approximation of
matrix rank. In the streaming model, sketches of input matrices are
maintained under updates of matrix entries; we prove results for turnstile
updates, given in an arbitrary order. We give the first lower bounds known
for the space needed by the sketches, for a given estimation error
$\epsilon$. We sharpen prior upper bounds, with respect to combinations of
space, failure probability, and number of passes. The sketch we use for
matrix $A$ is simply $S^TA$, where $S$ is a sign matrix. Our results include
the following upper and lower bounds on the bits of space needed for $1$-pass
algorithms. Here $A$ is an $n\times d$ matrix, $B$ is an $n\times d'$ matrix,
and $c := d+d'$. These results are given for fixed failure probability; for
failure probability $\delta>0$, the upper bounds require a factor of
$\log(1/\delta)$ more space. We assume the inputs have integer entries
specified by $O(\log(nc))$ bits, or $O(\log(nd))$ bits. The Frobenius matrix
norm is used.

- (Matrix Product)
- Output matrix $C$ with $$|| A^TB-C || \leq \epsilon ||A|| ||B||.$$ We show that $\Theta(c\epsilon^{-2}\log(nc))$ space is needed.
- (Linear Regression)
- For $d'=1$, so that $B$ is a vector $b$, find $x$ so that $$||Ax-b|| \leq (1+\epsilon)\min_{x' \in \reals^d} || Ax'-b ||.$$ We show that $\Theta(d^2\epsilon^{-1}\log(nd))$ space is needed.
- (Rank-$k$ Approximation)
- Find matrix $\hat A_k$ of rank no more than $k$, so that $$|| A - \hat A_k|| \leq (1+\epsilon)||A-A_k||,$$ where $A_k$ is the best rank-$k$ approximation to $A$. Our lower bound is $\Omega(k\epsilon^{-1}(n+d)\log(nd))$ space, and we give a one-pass algorithm matching this when $A$ is given row-wise or column-wise. For general updates, we give a one-pass algorithm needing $$ O(k\epsilon^{-2}(n + d/\epsilon^2)\log(nd)) $$ space. We also give upper and lower bounds for algorithms using multiple passes, and a bicriteria low-rank approximation.

Survey talk given at SoCG; please see later slides mostly incorporating this
material, 2008.

We investigate ways in which an
algorithm can improve its expected performance by fine-tuning itself
automatically with respect to an unknown input distribution $D$. We assume
here that $D$ is of product type. More precisely, suppose that we need to
process a sequence $I_1, I_2,\ldots$ of inputs $ I = (x_1, x_2, \ldots, x_n)$
of some fixed length $n$, where each $x_i$ is drawn independently from some
arbitrary, unknown distribution $D_i$. The goal is to design an algorithm for
these inputs so that eventually the expected running time will be optimal for
the input distribution $D = D_1 * D_2 * ... * D_n$. We give such
self-improving algorithms for two problems: (i) sorting a sequence of numbers
and (ii) computing the Delaunay triangulation of a planar point set. Both
algorithms achieve optimal expected limiting complexity. The algorithms begin
with a training phase during which they collect information about the input
distribution, followed by a stationary regime in which the algorithms settle
to their optimized incarnations.

We study the problem of
two-dimensional Delaunay triangulation in the self-improving algorithms
model. We assume that the `n`points of the input each come from an
independent, unknown, and arbitrary distribution. The first phase of our
algorithm builds data structures that store relevant information about the
input distribution. The second phase uses these data structures to
efficiently compute the Delaunay triangulation of the input. The running time
of our algorithm matches the information-theoretic lower bound for the given
input distribution, implying that if the input distribution has low entropy,
then our algorithm beats the standard `\Omega(n \log n)` bound for computing
Delaunay triangulations. Our algorithm and analysis use a variety of
techniques: `\epsilon`-nets for disks, entropy-optimal point-location data
structures, linear-time splitting of Delaunay triangulations, and
information-theoretic arguments.

In SoCG '08: Proceedings of the Twenty-Fourth Annual Symposium on
Computational Geometry, 2008.

The Johnson-Lindenstrauss random
projection lemma gives a simple way to reduce the dimensionality of a set of
points while approximately preserving their pairwise distances. The most
direct application of the lemma applies to a finite set of points, but recent
work has extended the technique to affine subspaces, curves, and general
smooth manifolds. Here the case of random projection of smooth manifolds is
considered, and a previous analysis is sharpened, reducing the dependence on
such properties as the manifold's maximum curvature.

- July 22, 2008: Various corrections (actually done March 2008)
- December 2007: First version

ACM Transactions on Algorithms, 6(4), 2010.

Preliminary version in SODA '08: Proceedings of the Nineteenth
Annual ACM-SIAM Symposium on Discrete Algorithms, 2008.
The problem of maximizing a concave function
$f(x)$ in a simplex $S$ can be solved approximately by a simple greedy
algorithm, that for given $k$ can find a point $x_{(k)}$ on a $k$-dimensional
face such that $f(x_{(k)}) \ge f(x_*) - O(1/k)$, where $f(x_*)$ is the
maximum value of $f$ in $S$. This algorithm and analysis were known before,
and related to problems of statistics and machine learning, such as boosting,
regression, and density mixture estimation. In other work, coming from
computational geometry, the existence of * $\epsilon$-coresets* was
shown for the minimum enclosing ball problem, by means of a simple greedy
algorithm. Similar greedy algorithms, that are special cases of the
Frank-Wolfe algorithm, were described for other enclosure problems. Here
these results are tied together, stronger convergence results are reviewed,
and several coreset bounds are generalized or strengthened.

- July 22, 2008: Various corrections (actually done October 2007)
- June 19, 2007: Corrected probabilistic existence proof (per K. Varadarajan), typos
- April 29, 2007: Clearer statement of results, cleaner proof for algorithms with away steps, typos
- March 26, 2007: First version

We give a model for the performance
impact on wireless systems of the limitations of certain resources, namely,
the base-station power amplifier and the available OVSF codes. These
limitations are readily modeled in the *loss model* formulation as a
*stochastic knapsack*. A simple and well-known recurrence of Kaufman
and Roberts allows the predictions of the model to be efficiently calculated.
We discuss the assumptions and approximations we have made that allow the use
of the model. We have included the model in Ocelot, a Lucent tool for
modeling and optimizing cellular phone systems. The model is fast to compute,
differentiable with respect to the relevant parameters, and able to model
broad ranges of capacity and resource use. These conditions are critical to
our application of optimization.

We investigate models for uplink
interference in wireless systems. Our models account for the effects of
outage probabilities. Such an accounting requires a nonlinear, even nonconvex
model, since increasing interference at the receiving base station increases
both mobile transmit power *and* outage probability, and this results
in a complex interaction. Our system model always has at least one solution,
a fixed point, and it is provably unique under certain reasonable conditions.
Our main purpose is to model real wireless systems as accurately as possible,
and so we test our models on realistic scenarios using data from a
sophisticated simulator. Our algorithm for finding a fixed point works very
well on such scenarios, and is guaranteed to find the fixed point when we can
prove it is unique. A slightly simplified model reduces the main data
structure for a $K$-sector market to $16K^2$ bytes of memory.

In STOC '06: Proceedings of the Thirty-Eighth Annual ACM Symposium
on Theory of Computing, 2006.

This work addresses the problem of
approximating a manifold by a simplicial mesh, and the related problem of
building triangulations for the purpose of piecewise-linear approximation of
functions. It has long been understood that the vertices of such meshes or
triangulations should be "well-distributed," or satisfy certain "sampling
conditions." This work clarifies and extends some algorithms for finding such
well-distributed vertices, by showing that they can be regarded as finding
*`\epsilon`-nets* or *Delone sets* in appropriate metric
spaces. In some cases where such Delone properties were already understood,
such as for meshes to approximate smooth manifolds that bound convex bodies,
the upper and lower bound results are extended to more general manifolds; in
particular, under some natural conditions, the minimum Hausdorff distance for
a mesh with `n` simplices to a `d`-manifold `M` is
$$\Theta((\int_M\sqrt{|\kappa(x)|}/n)^{2/d})$$ as `n\rightarrow\infty`, where
`\kappa(x)` is the Gaussian curvature at point `x\in M`. We also relate these
constructions to Dudley's approximation scheme for convex bodies, which can
be interpreted as involving an `\epsilon`-net in a metric space whose
distance function depends on surface normals. Finally, a novel scheme is
given, based on the Steinhaus transform, for scaling a metric space by a
Lipschitz function to obtain a new metric. This scheme is applied to show
that some algorithms for building finite element meshes and for surface
reconstruction can be also be interpreted in the framework of metric space
`\epsilon`-nets.

- Revised from 20/2/06: patches to upper bound proof, lower bound proof, many typos etc.
- Revised from version of 11/19/05: better upper bound proof, typos in Dudley example, cites peyre/cohen.

(Survey).

In G. Shakhnarovich, T. Darrell, and P. Indyk, editors,
Nearest-Neighbor Methods for Learning and Vision: Theory and
Practice, pages 15--59.
MIT Press, 2006.Given a set `S` of points in a metric
space with distance function `D`, the *nearest-neighbor searching*
problem is to build a data structure for `S` so that for an input query point
`q`, the point `s\in S` that minimizes `D(s,q)` can be found quickly. We
survey approaches to this problem, and its relation to concepts of metric
space dimension. Several measures of dimension can be estimated using
nearest-neighbor searching, while others can be used to estimate the cost of
that searching. In recent years, several data structures have been proposed
that are provably good for low-dimensional spaces, for some particular
measures of dimension. These and other data structures for nearest-neighbor
searching are surveyed.

Given a collection `S` of subsets of
some set `U`, and `M \subset U`, the *set cover* problem is to find
the smallest subcollection `C\subset S` such that `M` is a subset of the
union of the sets in `C`. While the general problem is NP-hard to solve, even
approximately, here we consider some geometric special cases, where usually
`U = \reals^d`. Combining previously known techniques [BG,CF], we show that
polynomial time approximation algorithms with provable performance exist,
under a certain general condition: that for a random subset `R\subset S` and
function `f()`, there is a decomposition of the complement
`U\setminus\cup_{Y\in R} Y` into an expected `f(|R|)` regions, each region of
a particular simple form. Under this condition, a cover of size `O(f(|C|))`
can be found in polynomial time. Using this result, and combinatorial
geometry results implying bounding functions `f(c)` that are nearly linear,
we obtain `o(\log c)` approximation algorithms for covering by fat triangles,
by pseudodisks, by a family of fat objects, and others. Similarly,
constant-factor approximations follow for similar-sized fat triangles and fat
objects, and for fat wedges. With more work, we obtain constant-factor
approximation algorithms for covering by unit cubes in `\reals^3`, and for
guarding an `x`-monotone polygonal chain.

In SODA '05: Proceedings of the Sixteenth Annual ACM-SIAM Symposium
on Discrete Algorithms, 2005.

Given an `n\times d` matrix `A` and an
`n`-vector `b`, the *`L_1` regression* problem is to find the vector
`x` minimizing the objective function `||Ax-b||_1`, where `||y||_1 \equiv
\sum_i |y_i|` for vector `y`. This paper gives an algorithm needing `O(n\log
n)d^{O(1)}` time in the worst case to obtain an approximate solution, with
objective function value within a fixed ratio of optimum. Given `\epsilon>0`,
a solution whose value is within `1+\epsilon` of optimum can be obtained
either by a deterministic algorithm using an additional
`O(n)(d/\epsilon)^{O(1)}` time, or by a Monte Carlo algorithm using an
additional `O((d/\epsilon)^{O(1)})` time. The analysis of the randomized
algorithm shows that weighted coresets exist for `L_1` regression. The
algorithms use the ellipsoid method, gradient descent, and random
sampling.

Let `S` be a set of `n` points in `d`
dimensions. A `k`-set of `S` is a subset of size `k` that is the intersection
of `S` with some open halfspace. This note shows that if the points of `S`
are random, with a coordinate-wise independent distribution, then the
expected number of `k`-sets of `S` is `O((k\log(en/k))^{d-1})2^d/(d-1)!`, as
`k\log n->\infty`, with a constant independent of the dimension.

We introduce a simple model of the effect
of temporal variation in signal strength on active-set membership, for
cellular phone systems that use the soft-handoff algorithm of IS-95a. This
model is based on a steady-state calculation, and its applicability is
confirmed by Monte Carlo studies.

Preliminary version presented at ALENEX99, 2003.

Given a set `S` of `n` *sites* (points),
and a distance measure `d`, the *nearest neighbor searching* problem
is to build a data structure so that given a query point `q`, the site
nearest to `q` can be found quickly. This paper gives a data structure for
this problem; the data structure is built using the distance function as a
"black box". The structure is able to speed up nearest neighbor searching in
a variety of settings, for example: points in low-dimensional or structured
Euclidean space, strings under Hamming and edit distance, and bit vector data
from an OCR application. The data structures are observed to need linear
space, with a modest constant factor. The preprocessing time needed per site
is observed to match the query time. The data structure can be viewed as an
application of a "kd-tree" approach in the metric space setting, using
Voronoi regions of a subset in place of axis-aligned boxes.

This paper gives an algorithm for solving
linear systems, using a randomized version of incomplete `LU` factorization
together with iterative improvement. The factorization uses Gaussian
elimination with partial pivoting, and preserves sparsity during
factorization by randomized rounding of the entries. The resulting
approximate factorization is then applied to estimate the solution. This
simple technique, combined with iterative improvement, is demonstrated to be
effective for a range of linear systems. When applied to medium-sized sample
matrices for PDEs, the algorithm is qualitatively like multigrid: the work
per iteration is typically linear in the order of the matrix, and the number
of iterations to achieve a small residual is typically on the order of
fifteen to twenty. The technique is also tested for a sample of asymmetric
matrices from the *Matrix Market*, and is found to have similar
behavior for many of them.

3G wireless systems such as 3G-1X, 1xEV-DO
and 1xEV-DV provide support for a variety of high-speed data applications.
The success of these services critically relies on the capability to ensure
an adequate QoS experience to users at an affordable price. With wireless
bandwidth at a premium, traffic engineering and network planning play a vital
role in addressing these challenges. We present models and techniques that we
have developed for quantifying the QoS perception of 1xEV-DO users generating
FTP or Web browsing sessions. We show how user-level QoS measures may be
evaluated by means of a Processor-Sharing model which explicitly accounts for
the throughput gains from multi-user scheduling. The model provides simple
analytical formulas for key performance metrics such as response times,
blocking probabilities and throughput. Analytical models are especially
useful for network deployment and in-service tuning purposes due to the
intrinsic difficulties associated with simulation-based optimization
approaches. We discuss the application of our results in the context of
Ocelot, which is a Lucent tool for wireless network planning and
optimization.

For 3G cellular networks, capacity is an
important objective, along with coverage, when characterizing the performance
of high-data-rate services. In live networks, the effective network capacity
heavily depends on the degree that the traffic load is balanced over all
cells, so changing traffic patterns demand dynamic network reconfiguration to
maintain good performance. Using a four-cell sample network, and antenna
tilt, cell power level and pilot fraction as adjustment variables, we study
the competitive character of network coverage and capacity in such a network
optimization process, and how it compares to the CDMA-intrinsic
coverage-capacity tradeoff driven by interference. We find that each set of
variables provides its distinct coverage-capacity tradeoff behavior with
widely varying and application-dependent performance gains. The study shows
that the impact of dynamic load balancing highly depends on the choice of the
tuning variable as well as the particular tradeoff range of operation.

We give a simple analytic model of coverage
probability for CDMA cellular phone systems under lognormally distributed
shadow fading. Prior analyses have generally considered the coverage
probability of a single antenna; here we consider the probability of coverage
by an ensemble of antennas, using some independence assumptions, but also
modeling a limited form of dependency among the antenna fades. We use the
Fenton-Wilkinson approach of approximating the external interference `I_0` as
lognormally distributed. We show that our model gives a coverage probability
that is generally within a few percent of Monte Carlo estimates, over a wide
regime of antenna strengths and other relevant parameters.

Given a set of points `P\subset R^d` and
value `\epsilon>0`, an *`\epsilon`-core-set* `S \subset P` has the
property that the smallest ball containing `S` is within `\epsilon` of the
smallest ball containing `P`. This paper shows that any point set has an
`\epsilon`-core-set of size `\lceil 1/\epsilon\rceil `, and this bound is
tight in the worst case. A faster algorithm given here finds an
`\epsilon`-core-set of size at most `2/\epsilon`. These results imply the
existence of small core-sets for solving approximate `k`-center clustering
and related problems. The sizes of these core-sets are considerably smaller
than the previously known bounds, and imply faster algorithms; one such
algorithm needs `O(d n/\epsilon+(1/\epsilon)^{5})` time to compute an
`\epsilon`-approximate minimum enclosing ball (1-center) of `n` points in `d`
dimensions. A simple gradient-descent algorithm is also given, for computing
the minimum enclosing ball in `O(d n / \epsilon^{2})` time. This algorithm
also implies slightly faster algorithms for computing approximately the
smallest radius `k`-flat fitting a set of points.

Given a set of points `P\subset
\reals^d` and value `\epsilon>0`, an *$\epsilon$-core-set* `S \subset
P` has the property that the smallest ball containing `S` is within
`\epsilon` of the smallest ball containing `P`. This paper shows that any
point set has an `\epsilon`-core-set of size `\lceil 1/\epsilon\rceil `, and
this bound is tight in the worst case. Some experimental results are also
given, comparing this algorithm with a previous one, and with a more
powerful, but slower one.

- Revised May 2006: Removed dependence on size of smallest ball, consider variant, etc.

We present an algorithm based on lattice
reduction for the fast decoding of diagonal differential modulation across
multiple antenna. While the complexity of the maximum likelihood algorithm is
exponential both in the number of antenna and the rate, the complexity of our
approximate lattice algorithm is polynomial in the number of antennas and the
rate. We show that the error performance of our lattice algorithm is very
close to the maximum likelihood algorithm.

Discrete and Computational Geometry, 22:63--93, 1999.

Preliminary version in STOC '97: Proceedings of the Twenty-Ninth
Annual ACM Symposium on Theory of Computing, 1997.
Given a set `S` of `n` sites (points), and a
distance measure `d`, the *nearest neighbor searching* problem is to
build a data structure so that given a query point `q`, the site nearest to
`q` can be found quickly. This paper gives data structures for this problem
when the sites and queries are in a metric space. One data structure, `D(S)`,
uses a divide-and-conquer recursion. The other data structure, `M(S,Q)`, is
somewhat like a skiplist. Both are simple and implementable. The data
structures are analyzed when the metric space obeys a certain sphere-packing
bound, and when the sites and query points are random and have distributions
with an exchangeability property. This property implies, for example, that
query point `q` is a random element of `S\cup\{q\}`. Under these conditions,
the preprocessing and space bounds for the algorithms are close to linear in
`n`. They depend also on the sphere-packing bound, and on the logarithm of
the *distance ratio* `\Upsilon(S)` of `S`, the ratio of the distance
between the farthest pair of points in `S` to the distance between the
closest pair. The data structure `M(S,Q)` requires as input data an
additional set `Q`, taken to be representative of the query points. The
resource bounds of `M(S,Q)` have a dependence on the distance ratio of `S\cup
Q`. While `M(S,Q)` can return wrong answers, its failure probability can be
bounded, and is decreasing in a parameter `K`. Here `K\leq |Q|/n` is chosen
when building `M(S,Q)`. The expected query time for `M(S,Q)` is `O(K\log
n)\log\Upsilon(S\cup Q)`, and the resource bounds increase linearly in `K`.
The data structure `D(S)` has expected `O(\log n)^{O(1)}` query time, for
fixed distance ratio. The preprocessing algorithm for `M(S,Q)` can be used to
solve the all-nearest-neighbor problem for `S` in `O(n(\log
n)^2(\log\Upsilon(S))^2)` expected time.

Lecture notes, 1997.

The technique of randomized incremental
construction allows a variety of geometric structures to be built quickly.
This note shows that once such a structure is built, it is possible to store
the geometric input data for it so that the structure can be built again by a
randomized algorithm even more quickly. Except for the randomization, this
generalizes the technique of Snoeyink and van Kreveld that applies to planar
problems.

Given points moving with constant, but
possibly different, velocities, the minimum moving diameter problem is to
find the minimum, over all time, of the maximum distance between a pair of
points at each moment. This note gives a randomized algorithm requiring `O(n
\log n )` expected time for this problem, in two and three dimensions. Also
briefly noted is a randomized `O(n\log n\log\log n)` expected-time algorithm
for the discrete 1-center problem in three dimensions; in this problem, a
member `p` of a set `S` of points is desired, whose maximum distance to `S`
is minimum over all points of `S`.

In FOCS '94: Proceedings of the Thirty-Fifth Symposium on
Foundations of Computer Science, pages 695--702, 1994.

A simple idea for speeding up the computation
of extrema of a partially ordered set turns out to have a number of
interesting applications in geometric algorithms; the resulting algorithms
generally replace an appearance of the input size `n` in the running time by
an output size `A\leq n`. In particular, the `A` coordinate-wise minima of a
set of `n` points in `R^d` can be found by an algorithm needing `O(nA)` time.
Given `n` points uniformly distributed in the unit square, the algorithm
needs `n+O(n^{5/8})` point comparisons on average. Given a set of `n` points
in `R^d`, another algorithm can find its `A` extreme points in `O(nA)` time.
Thinning for nearest-neighbor classification can be done in time `O(n\log
n)\sum_i A_i n_i`, finding the `A_i` irredundant points among `n_i` points
for each class `i`, where `n=\sum_i n_i` is the total number of input points.
This sharpens a more obvious `O(n^3)` algorithm, which is also given here.
Another algorithm is given that needs `O(n)` space to compute the convex hull
of `n` points in `O(nA)` time. Finally, a new randomized algorithm finds the
convex hull of `n` points in `O(n\log A)` expected time, under the condition
that a random subset of the points of size `r` has expected hull complexity
`O(r)`. All but the last of these algorithms has polynomial dependence on the
dimension `d`, except possibly for linear programming.

Merges two papers below, 1994.

This paper gives an algorithm for
*polytope covering:* let `L` and `U` be sets of points in `R^d`,
comprising `n` points altogether. A *cover* for `L` from `U` is a set
`C\subset U` with `L` a subset of the convex hull of `C`. Suppose `c` is the
size of a smallest such cover, if it exists. The randomized algorithm given
here finds a cover of size no more than `c(5d\ln c)`, for `c` large enough.
The algorithm requires `O(c^2n^{1+\delta})` expected time. (In this paper,
`\delta` will denote any fixed value greater than zero.) More exactly, the
time bound is $$O(cn^{1+\delta}+c(nc)^{ 1/(1+\gamma/(1+\delta))}),$$ where
`\gamma\equiv 1/\lfloor d/2\rfloor`. The previous best bounds were `c O(\log
n)` cover size in `O(n^d)` time [MiS]. A variant algorithm is applied to the
problem of approximating the boundary of a polytope with the boundary of a
simpler polytope. For an appropriate measure, an approximation with error
`\epsilon` requires `c=O(1/\epsilon)^{(d-1)/2}` vertices, and the algorithm
gives an approximation with `c({5d^3\ln(1/\epsilon)})` vertices. The
algorithms apply ideas previously used for small-dimensional linear
programming. The final result here applies polytope approximation to the the
*post office problem*: given `n` points (called sites) in `d`
dimensions, build a data structure so that given a query point `q`, the
closest site to `q` can be found quickly. The algorithm given here is given
also a relative error bound `\epsilon`, and depends on a ratio `\rho`, which
is no more than the ratio of the distance between the farthest pair of sites
to the distance between the closest pair of sites. The algorithm builds a
data structure of size `O(n(\log\rho)/\epsilon^{d/2}` in time
`O(n^2(\log\rho))/\epsilon^d`. With this data structure, closest-point
queries can be answered in `O(\log n)/\epsilon^{d/2}` time.

In SoCG '94: Proceedings of the Tenth Annual Symposium on
Computational Geometry, pages 160--164, 1994.

In WADS '93: Proceedings of the Third Workshop on Algorithms and
Data Structures, pages 246--252, 1993.

We give a practical and provably good Monte
Carlo algorithm for approximating center points. Let `P` be a set of `n`
points in `\reals^d`. A point `c \in \reals^d` is a `\beta`-center point of
`P` if every closed halfspace containing `c` contains at least `\beta n`
points of `P`. Every point set has a `1/(d+1)`-center point; our algorithm
finds an `\Omega(1/d^2)`-center point with high probability. Our algorithm
has a small constant factor and is the first approximate center point
algorithm whose complexity is subexponential in `d`. Moreover, it can be
optimally parallelized to require `O(\log^2d\log\log n)` time. Our algorithm
has been used in mesh partitioning methods and can be used in constructing
high breakdown estimators for multivariate datasets in statistics. It has the
potential to improve results in practice for constructing weak
`\epsilon`-nets. We derive a variant of our algorithm whose time bound is
fully polynomial in `d` and linear in `n`, and show how to combine our
approach with previous techniques to compute high quality center points more
quickly.

Presentation at Fifth MSI-Stony Brook Workshop on Computational Geometry,
1996.

This talk sketches: a convenient method
for computing the volumes of Voronoi regions; a proof that "area-stealing"
natural neighbor interpolation works; a scheme for smoother natural neighbor
interpolation alternative to Sibson's method; the interpolation scheme used
in the "finite volume element" method; and the observation that the minimax
piecewise-linear interpolant of a convex function is the (lower) convex
hull.

In FOCS '92: Proceedings of the Thirty-First Symposium on
Foundations of Computer Science, pages 387--395, Pittsburgh, PA,
October 1992.

The problem of evaluating the sign of the
determinant of a small matrix arises in many geometric algorithms. Given an
`n\times n` matrix `A` with integer entries, whose columns are all smaller
than `M` in Euclidean norm, the algorithm given here evaluates the sign of
the determinant `\det A` exactly. The algorithm requires an arithmetic
precision of less than `1.5n+2\lg M` bits. The number of arithmetic
operations needed is `O(n^3)+O(n^2)\log\OD(A)/\beta`, where `\OD(A)|\det A|`
is the product of the lengths of the columns of `A`, and `\beta` is the
number of "extra" bits of precision, $$\min\{\lg(1/\mathbf{u})-1.1n-2\lg
n-2,\lg N - \lg M - 1.5n - 1\},$$ where `\mathbf{u}` is the roundoff error in
approximate arithmetic, and `N` is the largest representable integer. Since
`\OD(A)\leq M^n`, the algorithm requires `O(n^3\lg M)` time, and `O(n^3)`
time when `\beta=\Omega(\log M)`.

Discrete and Computational Geometry, 10:227--233,
1993.

This paper shows that the `i`-level of an
arrangement of hyperplanes in `E^d` has at most `\binom{i+d-1}{d-1}` local
minima. This bound follows from ideas previously used to prove bounds on
`(\le k)`-sets. Using linear programming duality, the Upper Bound Theorem is
obtained as a corollary, giving yet another proof of this celebrated bound on
the number of vertices of a simple polytope in `E^d` with `n` facets.

(Survey).

In F.~K. Hwang and D.~Z. Hu, editors, Computers and Euclidean
Geometry.
World Scientific Publishing, 1992.This paper surveys some of the applications of
randomization to computational and combinatorial geometry. Randomization
provides a general way to divide-and-conquer geometric problems, and gives a
simple incremental approach to building geometric structures. The paper
discusses closest-point problems, convex hulls, Voronoi diagrams, trapezoidal
diagrams of line segments, linear programming in small dimension, range
queries, and bounds for point-line incidences and for `(\le k)`-sets.
Relations to the Vapnik-Chervonenkis dimension, PAC-learnability of geometric
concepts, and the Hough transform are briefly noted.

We prove four results on randomized
incremental constructions (RICs):

- an analysis of the expected behavior under insertion and deletions,
- a fully dynamic data structure for convex hull maintenance in arbitrary dimensions,
- a tail estimate for the space complexity of RICs,
- a lower bound on the complexity of a game related to RICs.

We describe randomized parallel algorithms for
building trapezoidal diagrams of line segments in the plane. The algorithms
are designed for a CRCW PRAM. For general segments, we give an algorithm
requiring optimal `O(A+n\log n)` expected work and optimal `O(\log n)` time,
where `A` is the number of intersecting pairs of segments. If the segments
form a simple chain, we give an algorithm requiring optimal `O(n)` expected
work and `O(\log n\log\log n\log^*n)` expected time, and a simpler algorithm
requiring `O(n\log^*n)` expected work. The serial algorithm corresponding to
the latter is among the simplest known algorithms requiring `O(n\log^*n)`
expected operations. For a set of segments forming `K` chains, we give an
algorithm requiring `O(A+n\log^*n+K\log n)` expected work and `O(\log
n\log\log n\log^* n)` expected time. The parallel time bounds require the
assumption that enough processors are available, with processor allocations
every `\log n` steps.

In SODA '91: Proceedings of the Second Annual ACM-SIAM Symposium on
Discrete Algorithms, January 1991.

This paper gives a partitioning scheme for the
geometric, planar traveling salesman problem, under the Euclidean metric:
given a set `S` of `n` points in the plane, find a shortest closed tour
(path) visiting all the points. The scheme employs randomization, and gives a
tour that can be expected to be short, if `S` satisfies the condition that a
random subset `R\subset S` has on average a tour much shorter than an optimal
tour of `S`. This condition holds for points independently, identically
distributed in the plane, for example, for which a tour within `1+\epsilon`
of shortest can be found in expected time `nk^2 2^k`, where `k=O(\log\log
n)^3/\epsilon^2`. One algorithm employed in the scheme is of interest in its
own right: when given a simple polygon `P`, it finds a Steiner triangulation
of the interior of `P`. If `P` has `n` sides and perimeter `L_P`, the edges
of the triangulation have total length `L_PO(\log n)`. If this algorithm is
applied to a simple polygon induced by a minimum spanning tree of a point
set, the result is a Steiner triangulation of the set with total length
within a factor of `O(\log n)` of the minimum possible.

Journal of the ACM, 42(2):488--499, 1995.

Preliminary version in FOCS '88: Proceedings of the Twenty-Ninth
Symposium on Foundations of Computer Science, 1988.
This paper gives an algorithm for solving
linear programming problems. For a problem with `n` constraints and `d`
variables, the algorithm requires an expected $$O(d^2n)+(\log
n)O(d)^{d/2+O(1)} +O(d^4\sqrt{n}\log n)$$ arithmetic operations, as
`n\rightarrow\infty`. The constant factors do not depend on `d`. Also, an
algorithm is given for integer linear programming. Let `\varphi` bound the
number of bits required to specify the rational numbers defining an input
constraint or the objective function vector. Let `n` and `d` be as before.
Then the algorithm requires expected $$O(2^ddn+8^dd\sqrt{n\ln n}\ln n)
+d^{O(d)}\varphi\ln n$$ operations on numbers with `d^{O(1)}\varphi` bits, as
`n->oo`, where the constant factors do not depend on `d` or `\varphi`. The
expectations are with respect to the random choices made by the algorithms,
and the bounds hold for any given input. The technique can be extended to
other convex programming problems. For example, an algorithm for finding the
smallest sphere enclosing a set of `n` points in `E^d` has the same time
bound.

(The accompanying talk is the one given at FOCS in 1988; note that it gives the results of the paper in terms of the Smallest Enclosing Sphere (or Minimum Enclosing Ball) problem.)

The delay between conference and journal publication is not the fault of the journal.

Developments between 1988 and 1995, roughly as discussed at the conclusion of the journal version of the paper:

Several developments have occurred since the conference version of this paper appeared. Adler and Shamir have shown that these ideas can be applied to general convex programming. Chazelle and Matou{\v s}ek have derandomized the recursive algorithm, obtaining a deterministic algorithm requiring `d^{O(d)}n` time. Alon and Megiddo have applied and extended the ideas of this paper to a parallel setting. Seidel gave a different randomized algorithm, requiring `O(d!n)` expected time, with a somewhat simpler analysis; Matousek, Sharir and Welzl found a variant of Seidel's algorithm requiring time subexponential in `d`. Their algorithm is a randomized instance of the simplex algorithm. Kalai was the first to find a subexponential simplex algorithm. Problem instances have long been known for which versions of the simplex algorithm require at least `2^d` operations.[KM] These results cast new light on the complexity of the simplex algorithm, and on the possibility that linear programming problems can be solved in "strongly polynomial" time; such an algorithm would need `(nd)^{O(1)}` operations, with the number of operations independent of the size of the numbers specifying a problem instance.Some more recent related results: Vapnik's leave-one-out error estimate for support vector machines is a version of Lemma 3.2, generalized to quadratic programming. (Such

Chazelle
*et al.* observe that one of these algorithms can be the basis for
*sublinear* geometric algorithms; another paper observes that the approach
works well from the standpoint of *multi-pass* algorithms. Lemma 3.2
(or its generalization to convex programming),
was rediscovered recently: Calafiore and Campi,
Theorem 1. Their proof rediscovers "backwards analysis".

We present an algorithm that
triangulates a simple polygon on `n` vertices in `O(n \log ^* n)` expected
time. The algorithm uses random sampling on the input, and its running time
does not depend on any assumptions about a probability distribution from
which the polygon is drawn.

We use random sampling for several new
geometric algorithms. The algorithms are "Las Vegas," and their expected
bounds are with respect to the random behavior of the algorithms. These
algorithms follow from new general results giving sharp bounds for the use of
random subsets in geometric algorithms. These bounds show that random subsets
can be used optimally for divide-and-conquer, and also give bounds for a
simple, general technique for building geometric structures incrementally.
One new algorithm reports all the intersecting pairs of a set of line
segments in the plane, and requires `O(A+n\log n)` expected time, where `A`
is the number of intersecting pairs reported. The algorithm requires `O(n)`
space in the worst case. Another algorithm computes the convex hull of `n`
points in `E^d` in `O(n\log n)` expected time for `d=3`, and `O(n^{\lfloor
d/2\rfloor})` expected time for `d>3`. The algorithm also gives fast expected
times for random input points. Another algorithm computes the diameter of a
set of `n` points in `E^3` in `O(n\log n)` expected time, and on the way
computes the intersection of `n` unit balls in `E^3`. We show that `O(n\log
A)` expected time suffices to compute the convex hull of `n` points in `E^3`,
where `A` is the number of input points on the surface of the hull.
Algorithms for halfspace range reporting are also given. In addition, we give
asymptotically tight bounds for `(\le k)`-sets, which are certain halfspace
partitions of point sets, and give a simple proof of Lee's bounds for high
order Voronoi diagrams.

In SoCG '88: Proceedings of the Fourth Annual Symposium on
Computational Geometry, Urbana, Illinois, June 1988.

Algorithmica, 4:461--469, 1989.

Included in PhD Thesis.
We describe an algorithm for finding a
minimum spanning tree of the weighted complete graph induced by a set of `n`
points in Euclidean `d`-space. The algorithm requires nearly linear expected
time for points that are independently uniformly distributed in the unit
`d`-cube. The first step of the algorithm is the spiral search procedure
described by Bentley, Weide, and Yao [BWY] for finding a supergraph of the
MST that has `O(n)` edges. (The constant factor in the bound depends on `d`.)
The next step is that of sorting the edges of the supergraph by weight using
a radix distribution, or "bucket," sort. These steps require linear expected
time. Finally, Kruskal's algorithm is used with the sorted edges, requiring
`O(n\alpha(cn,n))` time in the worst case, with `c>6`. Since the function
`\alpha(cn,n)` grows very slowly, this step requires linear time for all
practical purposes. This result improves the previous best `O(n\log\log^*n)`,
and employs a much simpler algorithm. Also, this result demonstrates the
robustness of bucket sorting, which requires `O(n)` expected time in this
case despite the probability dependency between the edge weights.

The problem of finding a rectilinear shortest
path amongst obstacles may be stated as follows: Given a set of obstacles in
the plane find a shortest rectilinear (`L_1`) path from a point `s` to a
point `t` which avoids all obstacles. The path may touch an obstacle but may
not cross an obstacle. We study the rectilinear shortest path problem for the
case where the obstacles are non-intersecting simple polygons, and present an
`O( n \log^2 n )` algorithm for finding such a path, where `n` is the number
of vertices of the obstacles. This algorithm requires `O(n \log n )` space.
Another algorithm is given that requires `O(n ( \log n ) ^{ 3/2} )` time and
space. We also study the case of rectilinear obstacles in three dimensions,
and show that `L_1` shortest paths can be found in `O( n^2 \log^ 3 n )`
time.

In STOC '87: Proceedings of the Nineteenth Annual ACM Symposium on
Theory of Computing, New York, New York, May 1987.

This paper gives approximation algorithms for
solving the following motion planning problem: Given a set of polyhedral
obstacles and points `s` and `t`, find a shortest path from `s` to `t` that
avoids the obstacles. The paths found by the algorithms are piecewise linear,
and the length of a path is the sum of the lengths of the line segments
making up the path. Approximation algorithms will be given for versions of
this problem in the plane and in three-dimensional space. The algorithms
return an `\epsilon`-short path, that is, a path with length within
`(1+\epsilon)` of shortest. Let `n` be the total number of faces of the
polyhedral obstacles, and `\epsilon` a given value satisfying
`0<\epsilon\leq\pi`. The algorithm for the planar case requires `O(n\log
n)/\epsilon` time to build a data structure of size `O(n/\epsilon)`. Given
points `s` and `t`, an `\epsilon`-short path from `s` to `t` can be found
with the use of the data structure in time `O(n/\epsilon+n\log n)`. The data
structure is associated with a new variety of Voronoi diagram. Given
obstacles `S\subset E^3` and points `s,t\in E^3`, an `\epsilon`-short path
between `s` and `t` can be found in
$$O(n^2\lambda(n)\log(n/\epsilon)/\epsilon^4 +n^2\log n\rho\log(n\log
\rho))$$ time, where `\rho` is the ratio of the length of the longest
obstacle edge to the distance between `s` and `t`. The function
`\lambda(n)=\alpha(n)^{O(\alpha(n)^{O(1)})}`, where the `\alpha(n)` is a form
of inverse of Ackermann's function. For `\log(1/\epsilon)` and `\log\rho`
that are `O(\log n)`, this bound is
`O(n^2(\log^2n)\lambda(n)/\epsilon^4)`.

Discrete and Computational Geometry, 2:195--222, 1987.

Preliminary version: Further applications of random sampling to
computational geometry, STOC '86: Proceedings of the Eighteenth Annual
ACM Symposium on Theory of Computing, 1986.
This paper gives several new demonstrations of
the usefulness of random sampling techniques in computational geometry. One
new algorithm creates a search structure for arrangements of hyperplanes by
sampling the hyperplanes and using information from the resulting arrangement
to divide and conquer. This algorithm requires `O(s ^ {d + \epsilon})`
expected preprocessing time to build a search structure for an arrangement of
`s` hyperplanes in `d` dimensions. The expectation, as with all expected
times reported here, is with respect to the random behavior of the algorithm,
and holds for any input. Given the data structure, and a query point `p`, the
cell of the arrangement containing `p` can be found in `O( \log s)`
worst-case time. (The bound holds for any fixed `\epsilon >0`, with the
constant factors dependent on `d` and `\epsilon`.) Using point-plane duality,
the algorithm may be used for answering halfspace range queries. Another
algorithm finds random samples of simplices to determine the separation
distance of two polytopes. The algorithm uses expected `O(n ^{\lfloor
d/2\rfloor} )` time, where `n` is the total number of vertices of the two
polytopes. This matches previous results [DoK] for the case `d=3` and extends
them. Another algorithm samples points in the plane to determine their order
`k` Voronoi diagram, and requires expected `O(s^{1+\epsilon}k)` time for `s`
points. (It is assumed that no four of the points are cocircular.) This
sharpens the bound `O(sk ^ 2 \log s)` for Lee's algorithm [Lee], and `O(s ^ 2
\log s + k(s-k) \log ^ 2 s)` for Chazelle and Edelsbrunner's algorithm [ChE].
Finally, random sampling is used to show that any set of `s` points in `E^3`
has `O(sk^2\log ^ 8 s / ( \log \log s) ^ 6 )` distinct `j`-sets with `j\leq
k`. (For `S \subset E^d`, a set `S'\subset S` with `|S'|=j` is a `j`-set of
`S` if there is a halfspace `h^+` with `S'=S \cap h^+`.) This sharpens with
respect to `k` the previous bound `O(s k ^ 5 )` [ChP]. The proof of the bound
given here is an instance of a "probabilistic method" [ErS].

Information Processing Letters, 22:21--24, January
1986.

SIAM Journal on Computing, pages 830--847, 1988.

Preliminary version: A probabilistic algorithm for the post office
problem, STOC '85: Proceedings of the Seventeenth Annual ACM Symposium
on Theory of Computing, 1985.
An algorithm for closest-point queries is
given. The problem is this: given a set `S` of `n` points in `d`-dimensional
space, build a data structure so that given an arbitrary query point `p`, a
closest point in `S` to `p` can be found quickly. The measure of distance is
the Euclidean norm. This is sometimes called the *post-office problem*
[Kn]. The new data structure will be termed an *RPO tree*, from
Randomized Post Office. The expected time required to build an RPO tree is
`O(n^{\lceil d/2\rceil (1+ \epsilon )} )`, for any fixed `\epsilon > 0`, and
a query can be answered in `O(\log n )` worst-case time. An RPO tree requires
`O(n^{\lceil d/2\rceil (1+ \epsilon )} )` space in the worst case. The
constant factors in these bounds depend on `d` and `\epsilon`. The bounds are
average-case due to the randomization employed by the algorithm, and hold for
any set of input points. This result approaches the `\Omega(n^{\lceil
d/2\rceil } )` worst-case time required for any algorithm that constructs the
Voronoi diagram of the input points, and is a considerable improvement over
previous bounds for `d>3`. The main step of the construction algorithm is the
determination of the Voronoi diagram of a random sample of the sites, and the
triangulation of that diagram.

This dissertation reports a variety of
new algorithms for solving closest-point problems. The input to these
algorithms is a set or sets of points in `d`-dimensional space, with an
associated `L_p` metric. The problems considered are:

In this work, approximation algorithms for some cases of these problems have been found. For example, for the minimum spanning tree problem with the `L_1` metric, an algorithm has been devised that requires `O(n \log^d (1/\rho))` time to find a spanning tree with weight within `1+\rho` of the minimum. Several other algorithms have been found with time bounds dependent on `\log(1/\rho)` for attaining error `\rho`.

Algorithms have also been found that require linear expected time, for independent identically distributed random input points with a probability density function satisfying weak conditions. One such algorithm depends on the fact that under certain conditions, values that are identically distributed, but dependent, can be bucket sorted in linear expected time.

An algorithm has been found for the all nearest neighbors problem that requires `O(n \log n)` expected time for any input set of points, where the expectation is on the random sampling performed by the algorithm. This algorithm involves the construction of a new data structure, a compressed form of digital trie.

- The all nearest neighbors problem. For point set `A`, find the nearest neighbors in `A` of each point in `A`.
- The nearest foreign neighbor problem. For point sets `A` and `B`, find the closest point in `B` to each point in `A`.
- The geometric minimum spanning tree problem. For point set `A`, find the minimum spanning tree for the complete weighted undirected graph associated with `A`, where the vertices of the graph correspond to the points of `A`, and the weight of an edge is the distance between the points defining the edge.

In this work, approximation algorithms for some cases of these problems have been found. For example, for the minimum spanning tree problem with the `L_1` metric, an algorithm has been devised that requires `O(n \log^d (1/\rho))` time to find a spanning tree with weight within `1+\rho` of the minimum. Several other algorithms have been found with time bounds dependent on `\log(1/\rho)` for attaining error `\rho`.

Algorithms have also been found that require linear expected time, for independent identically distributed random input points with a probability density function satisfying weak conditions. One such algorithm depends on the fact that under certain conditions, values that are identically distributed, but dependent, can be bucket sorted in linear expected time.

An algorithm has been found for the all nearest neighbors problem that requires `O(n \log n)` expected time for any input set of points, where the expectation is on the random sampling performed by the algorithm. This algorithm involves the construction of a new data structure, a compressed form of digital trie.

In STOC '84: Proceedings of the Sixteenth Annual ACM Symposium on
Theory of Computing, Washington, DC, April 1984.

Included in PhD Thesis.

In FOCS '83: Proceedings of the Twenty-Fourth Symposium on
Foundations of Computer Science, Tucson, AZ, November 1983.

Included in PhD Thesis.
We present new algorithms for the *all
nearest neighbors* problem:

Given a set `S` of `n` sites (points) in `d`-dimensional space, find the nearest neighbors in set `S` of each site in `S`.Our results:

- An algorithm for solving the all nearest neighbors problem in `O(n\log\sigma)` time, where `\sigma` is the ratio of the distance between the farthest pair of sites to the distance between the closest pair of sites. A similar algorithm is described for finding all `k`'th nearest neighbors.
- An algorithm
for building a
*celltree*, a compressed form of digital trie, in `O(n\log n)` probabilistic time. The logarithm, floor, and bitwise exclusive-or functions are assumed available at unit cost. - An algorithm for solving the all nearest neighbors problem in `O(n)` worst-case time, given a celltree for the sites.
- An algorithm for building a celltree in linear expected time, assuming the sites are independently identically distributed random variables, with an unknown probability density function obeying some very weak conditions.

Information Processing Letters, 16:23--25, January
1983.

(Survey).

In P. Cohen and E. Feigenbaum, editors, The Handbook of Artificial
Intelligence.
William Kaufman, Inc., Los Altos, CA, 1982.

In Proceedings of the DARPA Image Understanding Workshop,
Maclean, VA: Science Applications, Inc., 1981.

Technical report, Claremont Graduate School, Claremont, CA, 1977.

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