Enumeration of Rota-Baxter Words
  In this paper, we prove results on enumerations of sets of Rota-Baxter words in a finite number of generators and a finite number of unary operators. Rota-Baxter words are words formed by concatenating generators and images of words under Rota-Baxter operators. Under suitable conditions, they form canonical bases of free Rota-Baxter algebras and are studied recently in relation to combinatorics, number theory, renormalization in quantum field theory, and operads. Enumeration of a basis is often a first step to choosing a data representation in implementation. Our method applies some simple ideas from formal languages and compositions (ordered partitions) of an integer. We first settle the case of one generator and one operator where both have exponent 1 (the idempotent case). Some integer sequences related to these sets of Rota-Baxter words are known and connected to other combinatorial sequences, such as the Catalan numbers, and others are new. The recurrences satisfied by the generating series of these sequences prompt us to discover an efficient algorithm to enumerate the canonical basis of certain free Rota-Baxter algebras. More general sets of Rota-Baxter words are enumerated with summation techniques related to compositions of integers. 
Li Guo and William Y. Sit