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.
@article{118498, author = {Bernhard Banaschewski}, title = {Bourbaki's Fixpoint Lemma reconsidered}, journal = {Commentationes Mathematicae Universitatis Carolinae}, volume = {33}, year = {1992}, pages = {303-309}, zbl = {0779.06004}, mrnumber = {1189661}, language = {en}, url = {http://dml.mathdoc.fr/item/118498} }
Banaschewski, Bernhard. Bourbaki's Fixpoint Lemma reconsidered. Commentationes Mathematicae Universitatis Carolinae, Tome 33 (1992) pp. 303-309. http://gdmltest.u-ga.fr/item/118498/
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