Full Idea
The axioms of propositional logic are: A→(B→A); A→(B→C)→(A→B)→(A→C) ; and (¬A→¬B)→(B→A).
Gist of Idea
There are three axiom schemas for propositional logic
Source
Rod Girle (Modal Logics and Philosophy [2000], 6.5)
Book Reference
Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.92