Mathematician.

Andrey (Andrei) Andreyevich Markov (14 June 1856 N.S. – 20 July 1922) was a Russian mathematician. He is best known for his work on stochastic processes. A primary subject of his research later became known as Markov chains and Markov processes.

Markov and his younger brother Vladimir Andreevich Markov (1871–1897) proved Markov brothers' inequality. His son, another Andrei Andreevich Markov (1903–1979), was also a notable mathematician, making contributions to constructive mathematics and recursive function theory.

Sources:

1.) "Of course, Markov, an atheist and eventual excommunicate of the Church quarreled endlessly with his equally outspoken counterpart Nekrasov. The disputes between Markov and Nekrasov were not limited to mathematics and religion, they quarreled over political and philosophical issues as well." Gely P. Basharin, Amy N. Langville, Valeriy A. Naumov, The Life and Work of A. A. Markov, page 6.

2.) Naming Infinity: A True Story of Religious Mysticism and Mathematical Creativity. Harvard University Press. 2009. p. 69. ISBN 978-0-674-03293-4. "Markov (1856–1922), on the other hand, was an atheist and a strong critic of the Orthodox Church and the tzarist government (Nekrasov exaggeratedly called him a Marxist)."