On Final Coalgebras of Power-Set Functors and Saturated Trees - To George Janelidze on the Occasion of His Sixtieth Birthday

The final coalgebra for the finite power-set functor was described by Worrell who also proved that the final chain converges in ω + ω steps. We describe the step ω as the set of saturated trees, a concept equivalent to the modally saturated trees introduced by K. Fine in the 1970s in his study of modal logic. And for the bounded power-set functors Pλ, where λ is an infinite regular cardinal, we prove that the construction needs precisely λ+ω steps. We also generalize Worrell’s result to M -labeled trees for a commutative monoid M , yielding a final coalgebra for the corresponding functor Mf studied by H.-P. Gumm and T. Schröder. We describe the final chain of the power-set functor by introducing the concept of i-saturated tree for all ordinals i, and then prove that for i of cofinality ω, the i-th step in the final chain consists of all i-saturated, strongly extensional trees.