Catalan numbers as discrepancies for a family of substitutions on infinite alphabets

TL;DR

This paper investigates substitutions on infinite alphabets, revealing unique growth behaviors in tile counting functions that involve Catalan numbers, contrasting with finite alphabet cases.

Contribution

It introduces a new class of substitutions on infinite alphabets and links their discrepancy in tile counting to Catalan numbers, showing fundamentally different second-order growth.

Findings

Second term growth is at least exponential divided by fractional powers of n

Discrepancy expressed as linear combinations of Catalan numbers

Distinct growth behaviors from finite alphabet substitutions

Abstract

In this work, we consider a class of substitutions on infinite alphabets and show that they exhibit a growth behaviour which is impossible for substitutions on finite alphabets. While for both settings the leading term of the tile counting function is exponential (and guided by the inflation factor), the behaviour of the second-order term is strikingly different. For the finite setting, it is known that the second term is also exponential or exponential times a polynomial. We exhibit a large family of examples where the second term is at least exponential in $n$ divided by half-integer powers of $n$, where $n$ is the number of substitution steps. In particular, we provide an identity for this discrepancy in terms of linear combinations of Catalan numbers.