Coherence requirements for pseudo-algebras for the 2-monad (-)^2 on CAT

Bob Rosebrugh – 20 March 2002

The 2-monad (-)^2 on CAT has unit I described by identities, and multiplication C described by composition. For a category K, a functor F : K2 --> K satisfying FI = 1 admits a unique, (normal) pseudo-algebra structure for (-)2 if and only if there is any natural isomorphism F(F2) --> FC. When this is the case, the unique coherent natural isomorphism satisfies a simple equation. Moreover, the set of all natural transformations F(F2)--> FC forms a commutative monoid isomorphic to the centre of K. As Korostenski and Tholen showed, such `factorization algebras' are of interest as they correspond to factorization systems on K. This is joint work with Richard Wood