Page "Entscheidungsproblem" Paragraph 4

Before the question could be answered, the notion of " algorithm " had to be formally defined.

This was done by Alonzo Church in 1936 with the concept of " effective calculability " based on his λ calculus and by Alan Turing in the same year with his concept of Turing machines.

It was recognized immediately by Turing that these are equivalent models of computation.