On the combinatorics of the Hopf algebra of dissection diagrams

TL;DR

This paper investigates the Hopf algebra of dissection diagrams, exploring its coalgebra structure, conjecturing cofreedom properties, and analyzing associated pre-Lie structures, especially focusing on the algebra generated by degree 1 diagrams.

Contribution

It introduces a detailed study of the Hopf algebra of dissection diagrams, including conjectures on cofreedom, and examines the pre-Lie algebra structures related to these diagrams.

Findings

The Hopf algebra is a free commutative right-sided combinatorial Hopf algebra.

The pre-Lie algebra generated by degree 1 dissection diagrams is not free.

A Hopf algebra morphism between Grossman-Larson and the dual of the dissection diagram Hopf algebra is expressed using pre-Lie structures.

Abstract

In this article, we are interested in the Hopf algebra $\mathcal{H}_{D}$ of dissection diagrams introduced by Dupont in his thesis. We use the version with a parameter $x\in\mathbb{K}$. We want to study its underlying coalgebra. We conjecture it is cofree, except for a countable subset of $\mathbb{K}$. If $x=-1$ then we know there is no cofreedom. We easily see that $\mathcal{H}\_{D}$ is a free commutative right-sided combinatorial Hopf algebra according to Loday and Ronco. So, there exists a pre-Lie structure on its graded dual. Furthermore ${\mathcal{H}_{D}}^{\circledast}$ and the enveloping algebra of its primitive elements are isomorphic. Thus, we can equip ${\mathcal{H}\_{D}}^{\circledast}$ with a structure of Oudom and Guin. We focus on the pre-Lie structure on dissection diagrams and in particular on the pre-Lie algebra generated by the dissection diagram of degree $1$. We prove that it is not free. We express a Hopf algebra morphism between the Grossman and Larson Hopf algebra and ${\mathcal{H}_{D}}^{\circledast}$ by using pre-Lie and Oudom and Guin structures.