On the heat kernel of a Cayley graph of $\operatorname{PSL}_2\mathbb{Z}$

TL;DR

This paper derives an explicit formula for the heat kernel on the Cayley graph of the modular group, extending spectral methods and providing insights into the Laplace spectrum and spectral gaps.

Contribution

It introduces a novel spectral transfer approach to explicitly compute the heat kernel on the Cayley graph of _2, extending Chung--Yau's method and proposing a new conjecture on spectral gaps.

Findings

Explicit heat kernel formula for the Cayley graph of _2

Determination of the Laplace spectrum including eigenvalues and continuous spectrum

Numerical evidence supporting a conjecture on spectral gaps for _p groups

Abstract

In this paper, we obtain an explicit formula for the heat kernel on the infinite Cayley graph of the modular group $\operatorname{PSL}_2\mathbb{Z}$, given by the presentation $\langle a,b\mid a^2=1, b^3=1\rangle$. Our approach extends the method of Chung--Yau in~\cite{MR1667452} by observing that the Cayley graph strongly and regularly covers a weighted infinite line. We solve the spectral problem on this line to obtain an integral expression for its heat kernel, and then lift this to the Cayley graph using spectral transfer principles for strongly regular coverings. The explicit formula allows us to determine the Laplace spectrum, containing eigenvalues and continuous parts. As a by-product, we suggest a conjecture on the lower bound for the spectral gap of Cayley graphs of $\operatorname{PSL}_2\mathbb{F}_p$ with our generators, inspired by the analogy with Selberg's $1/4$-conjecture. Numerical evidence is provided for small primes.