`f(y) approx f(x) + (y-x)^T grad f(x)`
`max_{y in S} y^T grad f(x) = max_i grad f(x)_i = grad f(x)_{i'}`
`f{:(x:}_{:(k):}{:):} >= f(x_**) - C_f/(k+3)`
`w(x) := z(x) + f(x) - x^T grad f(x)`
`f(x):= x^Tb - x^TA^TAx = x^Tb - c^2`for MEB, where `c=Ax`
`z(x) + f(x) - x^T grad f(x)` | `= z(x) + (x^Tb - c^2) - x^T(b-2A^Tc) |
` = z(x) + c^2` |