Powersum Bases in Quasisymmetric Functions and Quasisymmetric Functions in Non-commuting Variables

TL;DR

This paper introduces a new combinatorially defined powersum basis for quasisymmetric functions and extends it to non-commuting variables, enriching the algebraic structure with shuffle products and coproducts.

Contribution

It presents a novel combinatorial powersum basis for quasisymmetric functions and extends it to non-commuting variables, with explicit algebraic operations and basis change rules.

Findings

New powersum basis refines symmetric powersum basis

Basis has shuffle product and deconcatenate coproduct

Extension to non-commuting variables with basis elements related to permutation bases

Abstract

We introduce a new $P$ basis for the Hopf algebra of quasisymmetric functions that refine the symmetric powersum basis. Unlike the quasisymmetric power sums of types 1 and 2, our basis is defined combinatorially: its expansion in quasisymmetric monomial functions is given by fillings of matrices. This basis has a shuffle product, a deconcatenate coproduct, and has a change of basis rule to the quasisymmetric fundamental basis by using tuples of ribbons. We lift our quasisymmetric powersum $P$ basis to the Hopf algebra of quasisymmetric functions in non-commuting variables by introducing fillings with disjoint sets. This new basis has a shifted shuffle product and a standard deconcatenate coproduct, and certain basis elements agree with the fundamental basis of the Malvenuto-Reutenauer Hopf algebra of permutations. Finally we discuss how to generalize these bases and their properties by using total orders on indices.