Everybody knows what a normal gabi-algebra is

Abstract

Let A be a k-algebra over a commutative ring k. By the renowned Tannaka-Kreĭn reconstruction, liftings of the monoidal structure from Mk to MA correspond to bialgebra structures on A and liftings of the closed monoidal structure correspond to Hopf algebra structures on A. In this paper, we determine conditions on A that correspond to liftings of the closed structure alone, i.e. without considering the monoidal one, which lead to the notion of what we call a gabi-algebra. First, we tackle the question from the general perspective of monads, then we focus on the set-theoretic and the linear setting. Our main and most surprising result is that a normal gabi-algebra, that is an algebra A whose category of modules is (associative and unital normal) closed with closed forgetful functor to Mk, is automatically a Hopf algebra (thus justifying our title).