Systems of Dyson-Schwinger equations

TL;DR

This paper characterizes solutions to systems of combinatorial Dyson-Schwinger equations within a Hopf algebra framework, identifying conditions under which these solutions generate Hopf subalgebras and describing their algebraic structures.

Contribution

It provides a complete classification of SDSE solutions that form Hopf subalgebras, introducing operations and families that generate all such solutions.

Findings

Characterization of all SDSE solutions generating Hopf subalgebras.

Introduction of operations transforming SDSE solutions.

Description of the associated Hopf algebra as dual to enveloping algebras of specific Lie algebras.

Abstract

We consider systems of combinatorial Dyson-Schwinger equations (briefly, SDSE) X_1=B^+_1(F_1(X_1,...,X_N))...X_N=B^+_N(F_N(X_1,...,X_N)) in the Connes-Kreimer Hopf algebra H_I of rooted trees decorated by I={1,...,N},where B^+_i is the operator of grafting on a root decorated by i, and F_1...,F_N are non-constant formal series.The unique solution X=(X_1,...,X_N) of this equation generates a graded subalgebra H_S of H_I. We characterize here all the families of formal series (F_1,...,F_N) such that H_S is a Hopf subalgebra. More precisely, we define three operations on SDSE (change of variables, dilatation and extension) and give two families of SDSE (cyclic and fundamental systems), and prove that any SDSE (S) such that H_S is Hopf is the concatenation of several fundamental or cyclic systems after the application of a change of variables, a dilatation and iterated extensions. We also describe the Hopf algebra H_S as the dual of the enveloping algebra of a Lie algebra g_S of one of the following types: 1. g_S is a Lie algebra of paths associated to a certain oriented graph. 2. g_S is an iterated extension of the Fa\`a di Bruno Lie algebra. 3. g_S is an iterated extension of an abelian Lie algebra.