1909 – 1994
Stephen Cole Kleene was an American mathematician and logician who made foundational contributions to recursion theory, intuitionistic logic, and the theory of computation. A student of Alonzo Church at Princeton, he helped establish the mathematical foundations of computer science and is considered one of the founders of theoretical computer science.
Founded recursion theory and introduced the Kleene hierarchy of arithmetical and analytical sets
Invented regular expressions and the Kleene star operator
Proved the recursion theorem (Kleene's fixed-point theorem)
Developed realizability interpretation for intuitionistic logic
Co-formulated the Church-Turing thesis and proved key results on computable functions
Turing's thesis is not susceptible to mathematical proof
claimThere is a fundamental tension between treating logical knowledge as a priori and the computational intractability of deciding logical validity.
claimThe universal function u_1(i,x) = g_i(x) for unary primitive recursive functions cannot itself be primitive recursive