Gödel's Completeness Theorem: Mathematical Logic, Provability First Order Kurt Gödel, Structure (Mathematical Logic), Model Theory, Proof Soundness Theorem

Paris Harrington Theorem: Mathematical Logic, Ramsey Theory, Peano Arithmetic, Gödel's First Incompleteness Theorem

Rosser''s Trick: Rosser's Theorem, Gödel's Incompleteness Theorems, Mathematical Logic, Omega Consistency

Sahlqvist Formula: First Order Logic, Modal Mathematical Logic

Cantor's First Uncountability Proof: Georg Cantor, Transcendental number, diagonal argument, Theoretical computer...