Towards cohomology of renormalization: bigrading the combinatorial Hopf algebra of rooted trees

Abstract

The renormalization of quantum field theory twists the antipode of a noncocommutative Hopf algebra of rooted trees, decorated by an infinite set of primitive divergences. The Hopf algebra of undecorated rooted trees, ${\mathcal{H}}_R$, generated by a single primitive divergence, solves a universal problem in Hochschild cohomology. It has two nontrivial closed Hopf subalgebras: the cocommutative subalgebra ${\mathcal{H}}_{\mathrm{ladder}}$ of pure ladder diagrams and the Connes-Moscovici noncocommutative subalgebra ${\mathcal{H}}_{\mathrm{CM}}$ of noncommutative geometry. These three Hopf algebras admit a bigrading by $n$, the number of nodes, and an index $k$ that specifies the degree of primitivity. In each case, we use iterations of the relevant coproduct to compute the dimensions of subspaces with modest values of $n$ and $k$ and infer a simple generating procedure for the remainder. The results for ${\mathcal{H}}_{\mathrm{ladder}}$ are familiar from the theory of partitions, while those for ${\mathcal{H}}_{\mathrm{CM}}$ involve novel transforms of partitions. Most beautiful is the bigrading of ${\mathcal{H}}_R$, the largest of the three. Thanks to Sloane's {\tt superseeker}, we discovered that it saturates all possible inequalities. We prove this by using the universal Hochschild-closed one-cocycle $B_{+}$, which plugs one set of divergences into another, and by generalizing the concept of natural growth beyond that entailed by the Connes-Moscovici case. We emphasize the yet greater challenge of handling the infinite set of decorations of realistic quantum field theory.

21 pages, LaTeX