copositive_inner.jl

export CopositiveInner

"""
    struct CopositiveInner{S} <: PolyJuMP.PolynomialSet
        # Inner approximation of the PSD cone, i.e. typically either
        # `SOSCone`, `DSOSCone` or `SDSOSCone`,
        psd_inner::S
    end

A symmetric matrix ``Q`` is copositive if ``x^{\\top} Q x \\ge 0`` for all
vector ``x`` in the nonnegative orthant. Checking copositivity is a
co-NP-complete problem [MK87] and this cone is only the inner approximation of
the cone of copositive symmetric matrices given by Minknowski sum of `psd_inner`
and the cone of symmetric matrices with nonnegative entries (the diagonal
entries can be chosen to be zero) [Lemma 3.164, BPT12].

The matrix with nonnegative entries can be interpreted as lagrangian
multipliers. For instance,
```julia
@polyvar x y
@constraint(model, x^2 - 2x*y + y^2 in CopositiveInner(SOSCone()))
```
is equivalent to
```julia
# Matrix that we require to be copositive
Q = [ 1 -1
     -1  1]
λ = @variable(model, lower_bound=0)
# Symmetric matrix of nonnegative entries
Λ = [0 λ
     λ 0]
using LinearAlgebra # For `Symmetric`
@constraint(model, Symmetric(Q - Λ) in PSDCone())
```
which is equivalent to
```julia
@polyvar x y
λ = @variable(model, lower_bound=0)
@constraint(model, x^2 - 2x*y + y^2 - 2*λ * x*y in SOSCone())
```
which is the same as, using the `domain` keyword,
```julia
@polyvar x y
@constraint(model, x^2 - 2x*y + y^2 in SOSCone(), domain = @set x*y ≥ 0)
```

As an important difference with its equivalent forms, the
[`GramMatrixAttribute`](@ref) for the copositive constraint is given by matrix
`Q` while for the equivalent form using the  domain` keyword, the value
of the attribute would correspond to the the gram matrix in the `psd_inner`
cone, i.e. which should be equal to `Q - Λ`.

[BPT12] Blekherman, G.; Parrilo, P. A. & Thomas, R. R.
*Semidefinite Optimization and Convex Algebraic Geometry*.
Society for Industrial and Applied Mathematics, **2012**.

[MK87] K. G. Murty and S. N. Kabadi.
*Some NP-complete problems in quadratic and nonlinear programming*.
Mathematical programming, 39:117–129, **1987**.
"""
struct CopositiveInner{S} <: SOSLikeCone
    psd_inner::S
end

function matrix_cone_type(::Type{CopositiveInner{S}}) where S
    return CopositiveInner{matrix_cone_type(S)}
end
function matrix_cone(set::Type{CopositiveInner{S}}, side_dimension) where S
    return CopositiveInner(matrix_cone(S, side_dimension))
end
side_dimension(set::CopositiveInner) = side_dimension(set.psd_inner)