Renormalization of quasisymmetric functions

TL;DR

This paper applies renormalization techniques from quantum field theory to define and analyze monomial quasisymmetric functions for all weak compositions, extending their algebraic structure and establishing a Hopf algebra isomorphism.

Contribution

It introduces a renormalization method for quasisymmetric functions that handles divergence issues and extends their algebraic framework to all weak compositions.

Findings

Defines monomial quasisymmetric functions for all weak compositions via renormalization.

Establishes an isomorphism between the algebra of renormalized quasisymmetric functions and the quasi-shuffle algebra.

Provides a power series realization of the free commutative Rota-Baxter algebra.

Abstract

As a natural basis of the Hopf algebra of quasisymmetric functions, monomial quasisymmetric functions are formal power series defined from compositions. The same definition applies to left weak compositions, while leads to divergence for other weak compositions. We adapt the method of renormalization in quantum field theory, in the framework of Connes and Kreimer, to deal with such divergency. This approach defines monomial quasisymmetric functions for any weak composition as power series while extending the quasi-shuffle (stuffle) relation satisfied by the usual quasisymmetric functions. The algebra of renormalized quasisymmetric functions thus obtained turns out to be isomorphic to the quasi-shuffle algebra of weak compositions, giving the former a natural Hopf algebra structure and the latter a power series realization. This isomorphism also gives the free commutative Rota-Baxter algebra a power series realization, in support of a suggestion of Rota that Rota-Baxter algebra should provide a broad context for generalizations of symmetric functions.