A related class of interest, particularly with reference to stability, is that of P ( − ) {\displaystyle P^{(-)}} -matrices, sometimes also referred to as N − P {\displaystyle N-P} -matrices. A matrix A is a P ( − ) {\displaystyle P^{(-)}} -matrix if and only if ( − A ) {\displaystyle (-A)} is a P-matrix (similarly for P 0 {\displaystyle P_{0}} -matrices). Since σ ( A ) = − σ ( − A ) {\displaystyle \sigma (A)=-\sigma (-A)} , the eigenvalues of these matrices are bounded away from the positive real axis.