# Hopf-Frobenius algebras and a new Drinfeld double

Joesph Collins (University of Strathclyde), Ross Duncan (University of Strathclyde)

The zx-calculus and related theories are based on so-called interacting Frobenius algebras, where a pair of dagger special commutative Frobenius algebras jointly form a pair of Hopf algebras. In this setting we introduce a generalisation of this structure, Hopf-Frobenius algebras, starting from a single Hopf algebra which is not necessarily commutative or cocommutative. We provide the necessary and sufficient condition for a Hopf algebra to be a Hopf-Frobenius algebra, and show that every Hopf algebra in the category of finite dimensional vector spaces is a Hopf-Frobenius algebra. Hopf-Frobenius algebras provide a notion of duality, and give us a “dual” Hopf algebra that is isomorphic to the usual dual Hopf algebra in a compact closed category. We use this isomorphism to construct a Hopf algebra isomorphic to the Drinfeld double that is defined on H \otimes H rather than H \otimes H*.