On some enumerative problems of lambda calculus

The article considers combinatorial problems associated with the enumeration of lambda-terms in a untyped lambda calculus, as well as in simply typed systems with a single atom in the style of Church. For the case of untyped lambda calculus a system of equations for generating functions is constructed which describes the number of lambda terms. In the case of typed lambda calculus, both the inhabited types and the simplest inhabitants in them are enumerated.