An effective criterion for multiple positive zeros of vertically parametrized polynomial systems

We present an effective criterion for determining whether a (augmented) vertically parametrized polynomial system admits multiple positive zeros for some choice of parameter values. Our method builds on previous algorithms from chemical reaction network theory and reduces the question to checking the feasibility of a linear system of equalities and inequalities. Our criterion provides a sufficient condition for the absence of multiple positive zeros that applies to any augmented vertically parametrized polynomial system, and we show that when the kernel of the coefficient matrix of the system displays a certain sparsity structure, this condition becomes also necessary. We give thereby a full characterization of the existence of multiple zeros for this type of systems.