Abstract:
We study theories based on the classical propositional logic. It follows from the lemma of Sushko's that for any classical propositional theory $T$ and substitution function $\varepsilon$ of formulas instead of propositional variables, the set $\varepsilon^{-1}(T)$ is also a classical propositional theory. In the paper, it is proved the following statement being more strong: for any consistent finitely axiomatized classical propositional theory $T$ there exists a substitution function $\varepsilon$ such that $T$ is a preimage of the set of all tautologies under $\varepsilon$. An algorithm of constructing of such a substitution function is given.
Keywords:lattice of theories of classical propositional logic, counterimages of substitutions, unification, Suszko's lemma.