1903 – 1979
Andrei Andreyevich Markov Jr. (1903–1979) was a Soviet mathematician and logician who made foundational contributions to constructive mathematics and the theory of algorithms. He developed the Markov algorithm formalism—a string-rewriting system equivalent in power to Turing machines—and led the Russian school of constructive mathematics. He maintained that the Church-Turing thesis is a philosophical conjecture about computability rather than a mathematical proposition and therefore cannot be formally proved.
Developed Markov algorithms, a string-rewriting formalism equivalent to Turing machines
Founded the Russian school of constructive (algorithmic) mathematics
Argued that the Church-Turing thesis is not susceptible to mathematical proof
Proved the undecidability of the word problem for semigroups
Authored foundational texts on the theory of algorithms