Background
Propositional logic begins with propositional variables, atomic units that represent concrete propositions. A formula consists of propositional variables connected by logical connectives in a meaningful way, so that the truth of the overall formula can be uniquely deduced from the truth or falsity of each variable. A valuation is a function that assigns each propositional variable either T (for truth) or F (for falsity). So, for example, using the propositional variables A and B, the binary connectives and representing disjunction and conjunction respectively, and the unary connective representing negation, the following formula can be obtained::. A valuation here must assign to each of A and B either T or F. But no matter how this assignment is made, the overall formula will come out true. For if the first conjunction is not satisfied by a particular valuation, then one of A and B is assigned F, which will cause the corresponding later disjunct to be T.
Read more about this topic: Tautology (logic)
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