A formula for $F$-Polynomials in terms of $C$-Vectors and Stabilization of $F$-Polynomials

Abstract

Given a quiver associated to a cluster algebra and a sequence of vertices, iterative mutation leads to $F$-Polynomials which appear in numerous places in the cluster algebraic literature. The coefficients of the monomials in these $F$-Polynomials are difficult to understand and have been an area of study for many years. In this paper, we present a general closed-form formula for these coefficients in terms of elementary manipulations with $C$-matrices. We then demonstrate the effectiveness of the formula by using it to derive simple explicit formulas for $F$-Polynomials of specific classes of quivers and mutation sequences. Work has been done to do these cases in ad-hoc combinatorial ways, but our formula recovers and improves known formulas with a general method. Secondly, we investigate convergence of $F$-polynomials. In themselves, they do not converge, but by changing bases using the $C$-matrix, they conjecturally do. Since our formula relates $C$-matrix entries to coefficients, we are able to apply it to make considerable progress on the conjecture. Specifically, we show stability for green mutation sequences. Finally, we look at exact formulas for these stable deformed $F$-polynomials in instances where they illustrate properties that the $F$-polynomials themselves do not.