Frobenius-Poincar\'e function and Hilbert-Kunz multiplicity

Abstract

We generalize the notion of Hilbert-Kunz multiplicity of a graded triple $(M,R,I)$ in characteristic $p>0$ by proving that for any complex number $y$, the limit $$\underset{n \to \infty}{\lim}(\frac{1}{p^n})^{\text{dim}(M)}\sum \limits_{j= -\infty}^{\infty}\lambda \left( (\frac{M}{I^{[p^n]}M})_j\right)e^{-iyj/p^n}$$ exists. We prove that the limiting function in the complex variable $y$ is entire and name this function the \textit{Frobenius-Poincar\'e function}. We establish various properties of Frobenius-Poincar\'e functions including its relation with the tight closure of the defining ideal $I$; and relate the study Frobenius-Poincar\'e functions to the behaviour of graded Betti numbers of $\frac{R}{I^{[p^n]}} $ as $n$ varies. Our description of Frobenius-Poincar\'e functions in dimension one and two and other examples raises questions on the structure of Frobenius-Poincar\'e functions in general.