Tautology Definition A proposition o of propostional logic is said to be valid if for any valuation v, v(o)=T. Such a proposition is also called a tautologyTautology . Definition . . A proposition σ of propostional logic is said to be valid if for any valuation V, V(σ) = T. Such a proposition is also called a tautology. Yi Li (Fudan University) Discrete Mathematics April 2, 2013 9 / 20