Questionnaire-2024/propositional_logic/theory_questions/4_2_notions_lect_sol.tex

Let $\varphi$ and $\psi$ be formulas in propositional logic. We say
that $\varphi$ and $\psi$ are semantically equivalent if and only if $\varphi \models \psi$ and $\psi \models \varphi$ holds.

In that case we write $\varphi \equiv \psi$.