Monday, 19 September 2011

ct.category theory - Are monads monadic?

In the book "Toposes, Triples and Theories", Barr and Wells study the question when a particular endofunctor admits a free monad. This is the case if the underlying category is complete and cocomplete and if the endofunctor preserves filtered colimits (we say that such a functor is finitary).



The question remains whether the resulting adjunction between monads on C and endofunctors on C is monadic. If C is locally finitely presentable (lfp), this is true: Steve Lack showed in



"On the monadicity of finitary monads", Journal of Pure and Applied Algebra, Volume 140, Number 1, 21 July 1999 , pp. 65-73(9) (available here)



that the forgetful functor mathrmMndf(C)rightarrowmathrmEndf(C) from finitary monads on a lfp category C to finitary endofunctors is monadic. Note that both these categories are again lfp categories, and we don't need universes to make sense of them, essentially because a finitary endofunctor is determined by what it does on a set of objects.



The result remains true if we consider categories and functors enriched in a complete and cocomplete symmetric monoidal closed category V which is lfp as a closed category.

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