Koga´s multivariable stability criterion

It is shown that a multivariable polynomial P(p;µ1, µ2,...,µs) is nonzero in Re p > 0, Re µ1> 0,..., Re µs> 0 if and only if P(p; 1, 1,..., 1) ≠ 0 and P(p; tanh τ1p, tanh τ2p,..., tanh τsp) ≠ 0 in Re p > 0 for all τ1> 0, τ2> 0,..., τs> 0.