# Tree series and pattern avoidance in syntax trees

## TL;DR

This paper introduces a general method for enumerating syntax trees that avoid certain patterns, using inclusion-exclusion formulas and operad theory, applicable without restrictions on the patterns.

## Contribution

It develops a novel approach combining pattern avoidance, formal power series, and operad structures for counting syntax trees in a unified framework.

## Key findings

- Method applies to any pattern set without restrictions.
- Connects syntax trees and operads for enumeration.
- Provides multiple concrete examples.

## Abstract

A syntax tree is a planar rooted tree where internal nodes are labeled on a graded set of generators. There is a natural notion of occurrence of contiguous pattern in such trees. We describe a way, given a set of generators $\mathfrak{G}$ and a set of patterns $\mathcal{P}$, to enumerate the trees constructed on $\mathfrak{G}$ and avoiding $\mathcal{P}$. The method is built around inclusion-exclusion formulas forming a system of equations on formal power series of trees, and composition operations of trees. This does not require particular conditions on the set of patterns to avoid. We connect this result to the theory of nonsymmetric operads. Syntax trees are the elements of such free structures, so that any operad can be seen as a quotient of a free operad. Moreover, in some cases, the elements of an operad can be seen as trees avoiding some patterns. Relying on this, we use operads as devices for enumeration: given a set of combinatorial objects we want enumerate, we endow it with the structure of an operad, understand it in term of trees and pattern avoidance, and use our method to count them. Several examples are provided.