Parametric Toricity of Steady State Varieties of Reaction Networks

TL;DR

This paper investigates the conditions under which steady state varieties of chemical reaction networks exhibit toricity or shifted toricity, considering parameter uncertainty and using real quantifier elimination methods.

Contribution

It extends the concept of toricity to parametric reaction networks and derives necessary and sufficient conditions for these properties to hold.

Findings

Shifted toricity is common in biological networks.

Toricity occurs only for degenerate parameter choices.

Parameters influence the algebraic structure of steady state varieties.

Abstract

We study real steady state varieties of the dynamics of chemical reaction networks. The dynamics are derived using mass action kinetics with parametric reaction rates. The models studied are not inherently parametric in nature. Rather, our interest in parameters is motivated by parameter uncertainty, as reaction rates are typically either measured with limited precision or estimated. We aim at detecting toricity and shifted toricity, using a framework that has been recently introduced and studied for the non-parametric case over both the real and the complex numbers. While toricity requires that the variety specifies a subgroup of the direct power of the multiplicative group of the underlying field, shifted toricity requires only a coset. In the non-parametric case these requirements establish real decision problems. In the presence of parameters we must go further and derive necessary and sufficient conditions in the parameters for toricity or shifted toricity to hold. Technically, we use real quantifier elimination methods. Our computations on biological networks here once more confirm shifted toricity as a relevant concept, while toricity holds only for degenerate parameter choices.