Automatic cocommutativity for the quantum-group comonad

Abstract

We prove that the cofree Hopf algebra on a cocommutative bialgebra is again cocommutative, and in fact coincides with the latter's largest Hopf subalgebra. This answers a question of Ardizzoni-Menini-Saracco and provides a proof for a statement to the same effect in the literature, whose original justification contains an apparent gap. The result relies on (and is in fact equivalent to) the observation that cocommutative bialgebra quotients of arbitrary Hopf algebras are automatically Hopf. This latter statement strengthens the analogue due to Newman, which assumes cocommutativity for the Hopf domain. One application of the above is the following quantum-rigidity result for Hopf algebras acted upon by a covariant quantum group: an action by one Hopf algebra on another which preserves the unital-algebra structure and the comultiplication factors through an action by a cocommutative Hopf algebra which moreover preserves the entire Hopf structure.