Sifted Colimits, Strongly Finitary Monads and Continuous Algebras

Abstract: We characterize strongly finitary monads on categories $\mathsf{Pos}$, $\mathsf{CPO}$ and $\mathsf{DCPO}$ as precisely those preserving sifted colimits. Or, equivalently, enriched finitary monads preserving reflexive coinserters. We study sifted colimits in general enriched categories. For $\mathsf{CPO}$ and $\mathsf{DCPO}$ we characterize varieties of continuous algebras as precisely the monadic categories for strongly finitary monads.