Eigenvalues of Real Matrices with Prescribed Principal Minors Sign and Descartes Law of Signs

In this paper, using Descartes law of sign, we provide elementary proof of results on the number of real eigenvalues of real matrices of which certain properties on the signs of their principal minors are known. More precisely, we analyze P, N, Q, R, PN and $QR$ matrices as well as their variants “almost”, “weak” and “sub-zero” matrices.