For 10 points, name this formal system created by Alonzo Church that heavily influenced the Lisp family of programming languages, in particular Scheme.
Two expressions in system are alpha-equivalent if they can be made literally the same by changing bound variables. The startup incubator Y Combinator is named after the fixed point operator in this language, used to create recursion. Since terms in the language consist entirely of variables and functions, numbers are implemented as "numerals" named after the language's inventor. Its inventor used the undecidability of equivalence in this Turing-complete model of computation to give a negative answer to Hilbert's Entscheidungsproblem in 1936, soon after Turing's own answer based on the Halting Problem. For 10 points, name this formal system created by Alonzo Church that heavily influenced the Lisp family of programming languages, in particular Scheme.
Two expressions in system are alpha-equivalent if they can be made literally the same by changing bound variables.
{Untyped Lambda calculus}