On computing the MIMO real structured stability margin

The problem of the robust stability of a polynomial multilinearly dependent on n independent bounded real uncertain parameters Δ_k ∈ [-1, +1] is considerable. In particular, it is shown by example that there may be a unique Δ=(Δ₁ , . . ., Δ_n) with precisely n-1 nonextremal elements for which the polynomial reaches zero. Hence, the conjecture that generically it suffices to consider Δ´s with at most two nonextremal elements is false