Improving an argument of Robin Moser, Messner and Thierauf used a construct named for this mathematician to give a constructive proof of the Lovasz local lemma. A result named for him can be used to show that there is a critical density in bond percolation; that result is his zero-one law. An equation co-named for this non-English mathematician can be used to compute n-step transition probabilities in Markov chains; that equation is also named for (*) Chapman. This man is the first namesake of a theorem that deals with 
Improving an argument of Robin Moser, Messner and Thierauf used a construct named for this mathematician to give a constructive proof of the Lovasz local lemma. A result named for him can be used to show that there is a critical density in bond percolation; that result is his zero-one law. An equation co-named for this non-English mathematician can be used to compute n-step transition probabilities in Markov chains; that equation is also named for (*) Chapman. This man is the first namesake of a theorem that deals with the preservation of quasiperiodic motion under small perturbations. His namesake complexity measure is used to prove Chaitin's incompleteness theorem and is sometimes called "descriptive complexity." The standard axiomatization of probability theory is due to, for 10 points, what Russian mathematician who names a theorem with Arnold and Moser?
Improving an argument of Robin Moser, Messner and Thierauf used a construct named for this mathematician to give a constructive proof of the Lovasz local lemma.
Andrey (Nikolaevich) Kolmogorov [accept Gregory Chaitin before “critical density”, since he sometimes co-names Kolmogorov complexity]