Entropy maximization

A derivative work by Judson Wilson, 6/2/2014.
Adapted from the CVX example of the same name, by Joëlle Skaf, 4/24/2008.

Introduction

Consider the linear inequality constrained entropy maximization problem: \begin{array}{ll} \mbox{maximize} & -\sum_{i=1}^n x_i \log(xi) \ \mbox{subject to} & \sum{i=1}^n x_i = 1 \ & Fx \succeq g, \end{array} where the variable is $x \in \mathbf{{\mbox{R}}}^{n}$.

This problem can be formulated in CVXPY using the entr atom.

Generate problem data

import cvxpy as cvx
import numpy as np

# Make random input repeatable. 
np.random.seed(0) 

# Matrix size parameters.
n = 20
m = 10
p = 5

# Generate random problem data.
tmp = np.mat(np.random.rand(n, 1))
A = np.mat(np.random.randn(m, n))
b = A*tmp
F = np.mat(np.random.randn(p, n))
g = F*tmp + np.mat(np.random.rand(p, 1))

Formulate and solve problem

# Entropy maximization.
x = cvx.Variable(n)
obj = cvx.Maximize(cvx.sum_entries(cvx.entr(x)))
constraints = [A*x == b,
               F*x <= g ]
prob = cvx.Problem(obj, constraints)
prob.solve(solver=cvx.CVXOPT, verbose=True)

# Print result.
print "\nThe optimal value is:", prob.value
print '\nThe optimal solution is:'
print x.value

     pcost       dcost       gap    pres   dres
 0:  0.0000e+00 -7.5220e+00  2e+01  1e+00  1e+00
 1: -6.0720e+00 -5.9875e+00  2e+00  1e-01  2e-01
 2: -5.4688e+00 -5.5885e+00  4e-01  2e-02  5e-02
 3: -5.4595e+00 -5.4889e+00  5e-02  2e-03  2e-02
 4: -5.4763e+00 -5.4816e+00  1e-02  3e-04  7e-03
 5: -5.4804e+00 -5.4809e+00  1e-03  4e-05  2e-03
 6: -5.4809e+00 -5.4809e+00  3e-05  1e-06  4e-04
 7: -5.4809e+00 -5.4809e+00  4e-07  1e-08  2e-05
 8: -5.4809e+00 -5.4809e+00  4e-09  1e-10  3e-07
 9: -5.4809e+00 -5.4809e+00  4e-11  1e-12  5e-09
Optimal solution found.

The optimal value is: 5.48090148635

The optimal solution is:
[[ 0.43483319]
 [ 0.66111715]
 [ 0.49201039]
 [ 0.36030618]
 [ 0.38416629]
 [ 0.30283658]
 [ 0.41730232]
 [ 0.79107794]
 [ 0.76667302]
 [ 0.38292365]
 [ 1.2479328 ]
 [ 0.50416987]
 [ 0.68053832]
 [ 0.67163958]
 [ 0.13877259]
 [ 0.5248668 ]
 [ 0.08418897]
 [ 0.56927148]
 [ 0.50000248]
 [ 0.78291311]]