Abstract:A constructively valid counterpart to Bourbaki's Fixpoint Lemma for chain-complete partially ordered sets is presented to obtain a condition for one closure system in a complete lattice $L$ to be stable under another closure operator of $L$. This is then used to deal with coproducts and other aspects of frames.
Keywords: complete lattice, closure operator, fixpoint, frame coproduct, compact frame
AMS Subject Classification: 06A15, 54A99, 54B99