  If a real polynomial $f(x)=p(x^2)+xq(x^2)$ is Hurwitz stable (every root if
$f$ lies in the open left half-plane), then the Hermite-Biehler Theorem says
that the polynomials $p(-x^2)$ and $q(-x^2)$ have interlacing real roots. We
extend this result to general polynomials by giving a lower bound on the number
of real roots of $p(-x^2)$ and $q(-x^2)$ and showing that these real roots
interlace. This bound depends on the number of roots of $f$ which lie in the
left half plane. Another classical result in the theory of polynomials is
Descartes' Rule of Signs, which bounds the number of positive roots of a
polynomial in terms of the number of sign changes in its coefficients. We use
our extension of the Hermite-Biehler Theorem to give an inverse rule of signs
for polynomials with one positive root.
