  In this contribution we solve the following problem. Let H_{nm} be a Hilbert
space of dimension nm, and let A be a positive semidefinite self-adjoint linear
operator on H_{nm}. Under which conditions on the spectrum has A a positive
partial transpose (is PPT) with respect to any partition H_n \otimes H_m of the
space H_{nm} as a tensor product of an n-dimensional and an m-dimensional
Hilbert space? We show that the necessary and sufficient conditions can be
expressed as a set of linear matrix inequalities on the eigenvalues of A.
