We consider the following problem: given k + 1 square matrices with ra
tional entries, A0, A1, ..., A(k), decide if A0 + r1A1 + ... + r(k)A(k
) is nonsingular for all possible choices of real numbers r1, ..., r(k
), in the interval [0, 1]. We show that this question, which is closel
y related to the robust stability problem, is NP-hard. The proof relie
s on the new concept of radius of nonsingularity of a square matrix an
d on the relationship between computing this radius and a graph-theore
tic problem.