Indefinite co positive matrices with exactly one positive eigenvalue or exactly one negative eigenvalue

Checking copositivity of a matrix is a co-NP-complete problem. This paper studies copositive matrices with certain spectral properties. It shows that an indefinite matrix with exactly one positive eigenvalue is copositive if and only if the matrix is nonnegative. Moreover, it shows that finding out if a matrix with exactly one negative eigenvalue is strictly copositive or not can be formulated as a combination of two convex quadratic programming problems which can be solved efficiently.