# Asymptotic Lech's inequality

## TL;DR

This paper investigates asymptotic versions of Lech's inequality in Noetherian local rings, establishing optimal bounds in characteristic p>0 and conjecturing their validity in all characteristics, with implications for Hilbert--Kunz multiplicity.

## Contribution

It proves optimal asymptotic bounds for Lech's inequality in characteristic p>0 and conjectures their extension to all characteristics, linking colength and Hilbert--Kunz multiplicity.

## Key findings

- For deep ideals, colength and Hilbert--Kunz multiplicity are arbitrarily close.
- Established bounds hold in characteristic p>0 for reduced, equidimensional rings with isolated singularities.
- Conjecture that these bounds are valid in all characteristics.

## Abstract

We explore the classical Lech's inequality relating the Hilbert--Samuel multiplicity and colength of an $\mathfrak{m}$-primary ideal in a Noetherian local ring $(R,\mathfrak{m})$. We prove optimal versions of Lech's inequality for sufficiently deep ideals in characteristic $p>0$, and we conjecture that they hold in all characteristics. Our main technical result shows that if $(R,\mathfrak{m})$ has characteristic $p>0$ and $\widehat{R}$ is reduced, equidimensional, and has an isolated singularity, then for any sufficiently deep $\mathfrak{m}$-primary ideal $I$, the colength and Hilbert--Kunz multiplicity of $I$ are sufficiently close to each other. More precisely, for all $\varepsilon>0$, there exists $N\gg0$ such that for any $I\subseteq R$ with $l(R/I)>N$, we have $(1-\varepsilon)l(R/I)\leq e_{HK}(I)\leq(1+\varepsilon)l(R/I)$.