Minimum ellipses

This example is from the Boyd & Vandenberghe book "Convex Optimization". Given a set of ellipses centered on the origin

E(A) = { u | u^T inv(A) u <= 1 }

find a "minimal" ellipse that contains the provided ellipses.

We can formulate this as an SDP:

minimize    trace(WX)
subject to  X >= A_i,    i = 1,...,m
            X PSD

where W is a PD matrix of weights to choose between different solutions.

using JuMP
import LinearAlgebra
import SCS
import Test

function example_min_ellipse()
    # We will use three ellipses: two "simple" ones, and a random one.
    As = [
        [2.0 0.0; 0.0 1.0],
        [1.0 0.0; 0.0 3.0],
        [2.86715 1.60645; 1.60645 1.12639],
    ]
    # We change the weights to see different solutions, if they exist
    weights = [1.0 0.0; 0.0 1.0]
    model = Model(SCS.Optimizer)
    set_silent(model)
    @variable(model, X[i = 1:2, j = 1:2], PSD)
    @objective(model, Min, LinearAlgebra.tr(weights * X))
    @constraint(model, [As_i in As], X >= As_i, PSDCone())
    optimize!(model)
    Test.@test termination_status(model) == OPTIMAL
    Test.@test primal_status(model) == FEASIBLE_POINT
    Test.@test objective_value(model) ≈ 6.46236 atol = 1e-4
    Test.@test value.(X) ≈ [3.1651 0.8022; 0.8022 3.2972] atol = 1e-4
    return
end

example_min_ellipse()