# CopositiveAnalyticCenter.jl

Solves problems of the form:

minimize    dot(obj, x)
subject to  norm(x) ≤ r
oracle(x) == true,


where obj is an AbstractVector, and oracle tests membership of some convex body. The main workhorse is the accp function, which solves this problem using an Analytic Center Cutting Plane method. It yields a near-optimal vector x.

For some vector x, oracle(x) should return true if x lies in the feasible set, or else a Halfspace containing the feasible set but not x. Halfspace(slope, constant) denotes the set {z: dot(slope, z) ≤ constant}.

For background information on this algorithm, see this preprint.

## Installation

Simply run

julia> import Pkg; Pkg.add("CopositiveAnalyticCenter")


This package will test if Gurobi.jl is installed properly. If not, ECOS.jl and Cbc.jl will be installed.

## Testing copositivity

The package provides the testcopositive function, which may be used in defining your oracle. For some symmetric matrix A, testcopositive(A) returns a Tuple containing the optimal value and optimal solution to

minimize    y' * A * y
subject to  sum(y) = 1
y ≥ 0.


To avoid having to set up the problem above from scratch every time testcopositive is called, you can create a CopositiveChecker instance cc, e.g. cc = CopositiveChecker(10); to set up an environment for testing 10-by-10 matrices. To test the 10-by-10 matrix A for copositivity using cc, call testcopositive(A, cc).

## Transforming vectors to symmetric matrices

To transform a vector to a symmetric matrix, you can use the vec2mat function included in the package. Its inverse is vec2matinv.

julia> using CopositiveAnalyticCenter

julia> A = vec2mat([1, 2, 3, 4, 5, 6])
3×3 Array{Int64,2}:
1  2  4
2  3  5
4  5  6

julia> vec2matinv(A)
6-element Array{Int64,1}:
1
2
3
4
5
6


The adjoint of vec2mat with respect to dot from LinearAlgebra.jl is vec2matadj.

julia> A = [1 2; 2 3];

3-element Array{Int64,1}:
1
4
3

julia> using LinearAlgebra: dot

julia> dot(vec2matadj(A), [0, 1, 0]) == dot(A, vec2mat([0, 1, 0]))
true


## Example usage

To solve the problem

minimize    dot(A, X)
subject to  norm(vec2matinv(X)) ≤ 1
X is copositive,


use the following function:

using CopositiveAnalyticCenter

function test_completely_positive(A)
# X = vec2mat(x), so dot(A, X) = dot(vec2matadj(A), x)
cc = CopositiveChecker(size(A,1))
function oracle(x::AbstractVector)
# Test if X = vec2mat(x) is copositive
val, y = testcopositive(vec2mat(x), cc)
if val >= 0
# If val ≥ 0, X = vec2mat(x) is copositive, so the oracle returns
# true.
return true
else
# Otherwise, y ≥ 0 satisfies y' X y < 0, while any copositive matrix
# lies in the halfspace
# {Z: y' Z y ≥ 0} = {vec2mat(z): dot(vec2matadj(-y*y'), z) ≤ 0}.

The package ships with the function completely_positive_cut which does the same thing as test_completely_positive above, but with some additional options.
Users interested in a yes-no answer to the question if A is completely positive can call is_completely_positive(A).