Lecture
This lecture introduces the concept of viewing proofs as directed acyclic graphs, where each step is represented by a node and directed edges. The instructor explains a minimal propositional logic proof system, defining formulas and inference rules. Various proofs for propositional logic formulas are demonstrated, highlighting the importance of soundness and completeness in a proof system. The lecture concludes with a discussion on the soundness and completeness of a proof system, emphasizing the significance of deriving all desired formulas and tautologies.