Quasi-shuffle products

TL;DR

This paper introduces quasi-shuffle products, a generalization of shuffle products, constructing a Hopf algebra structure on noncommutative polynomials, with applications to quasi-symmetric functions and algebraic deformations.

Contribution

It defines quasi-shuffle products for graded sets, constructs associated Hopf algebras, and explores their isomorphisms and deformations, extending the theory of shuffle algebras.

Findings

Quasi-shuffle products generalize shuffle products.

The resulting algebra forms a Hopf algebra structure.

Connections to quasi-symmetric functions and algebraic deformations.

Abstract

Given a locally finite graded set A and a commutative, associative operation on A that adds degrees, we construct a commutative multiplication * on the set of noncommutative polynomials in A which we call a quasi-shuffle product; it can be viewed as a generalization of the shuffle product. The resulting commutative algebra can be given the structure of a Hopf algebra (_A_,*,Delta). In the case where A is the set of positive integers and the operation on A is addition, (_A_,*,Delta) is the Hopf algebra of quasi-symmetric functions. If rational coefficients are allowed, there is a Hopf algebra isomorphism exp from the shuffle Hopf algebra on A onto (_A_,*,Delta). We discuss the dual of (_A_,*,Delta), and define a deformation *_q of * that coincides with * when q = 1 and is isomorphic to the concatenation product when q is not a root of unity. Finally, we discuss various examples of this construction.