Propositions Definition(Proposition) o Propositional letters are propositions o if a and B are propositions, then (aV3),(a^B),(-a),(a→B)and(a分)are propositions o A string of symbols is a proposition if and only if it can be obtained by starting with propositional letters(1)and repeatedly applying(2)Propositions Definition (Proposition) 1 Propositional letters are propositions. 2 if α and β are propositions, then (α ∨ β),(α ∧ β),(¬α),(α → β) and (α ↔ β) are propositions. 3 A string of symbols is a proposition if and only if it can be obtained by starting with propositional letters (1) and repeatedly applying (2). Yi Li (Fudan University) Discrete Mathematics March 20, 2012 8 / 24