Compactness in locales and in formal topology

If a locale is presented by a “flat site”, it is shown how its frame can be presented by generators and relations as a dcpo. A necessary and sufficient condition is derived for compactness of the locale (and also for its openness). Although its derivation uses impredicative constructions, it is also shown predicatively using the inductive generation of formal topologies. A predicative proof of the binary Tychonoff theorem is given, including a characterization of the finite covers of the product by basic opens. The discussion is then related to the double powerlocale. This is a preprint version of the article published as – Annals of Pure and Applied Logic 137 (2006), pp. 413–438. doi:10.1016/j.apal.2005.05.028