Consider the set of matrices A = {
}
Every matrix in the set A is Hurwitz if and only if there exist
, i = 1, 2, 3, 4, where Pi > 0,
i = 1, 2, 3, 4, such that

where
Equivalently, every matrix in the set A is Hurwitz if and only if there exist
, i = 1, 2, 3, 4,
where Qi > 0, i = 1, 2, 3, 4, such that
