Laplacians on Julia Sets III: Cubic Julia Sets and Formal Matings

TL;DR

This paper extends the construction of invariant Laplacians to cubic Julia sets and formal matings, revealing new spectral properties and employing numerical methods to analyze eigenvalues and eigenfunctions.

Contribution

It introduces a method for constructing Laplacians on cubic Julia sets and formal matings, with detailed analysis and numerical approximation of their spectra.

Findings

Eigenvalue multiplicities are even for cubic Julia sets.

Constructed Laplacians for specific cubic Julia sets and one mating example.

Spectral properties differ from quadratic Julia sets, explained by symmetry considerations.

Abstract

We continue the study of constructing invariant Laplacians on Julia sets, and studying properties of their spectra. In this paper we focus on two types of examples: 1) Julia sets of cubic polynomials $z^3 + c$ with a single critical point; 2) formal matings of quadratic Julia sets. The general scheme introduced in earlier papers in this series involves realizing the Julia set as a circle with identifications, and attempting to obtain the Laplacian as a renormalized limit of graph Laplacians on graphs derived form the circle with identifications model. In the case of cubic Julia sets the details follows the pattern established for quadratic Julia sets, but for matings the details are quite challenging, and we have only been completely successful for one example. Once we have constructed the Laplacian, we are able to use numerical methods to approximate the eigenvalues and eigenfunctions. One striking observation from the data is that for the cubic Julia sets the multiplicities of all eigenspaces (except for the trivial eigenspace of constants) are even numbers. Nothing like this is valid for the quadratic julia sets studied earlier. We are able to explain this, based on the fact that three is an odd number, and more precisely because the dihedral-3 symmetry group has only two distinct one-dimensional irreducible representations.