In control systems theory, the describing function (DF) method, developed by Nikolay Mitrofanovich Krylov and Nikolay Bogoliubov in the 1930s, and extended by Ralph Kochenburger is an approximate procedure for analyzing certain nonlinear control problems.
 It is based on quasi-linearization, which is the approximation of the non-linear system under investigation by a linear time-invariant (LTI) transfer function that depends on the amplitude of the input waveform.
 By definition, a transfer function of a true LTI system cannot depend on the amplitude of the input function because an LTI system is linear.
 Thus, this dependence on amplitude generates a family of linear systems that are combined in an attempt to capture salient features of the non-linear system behavior.
 The describing function is one of the few widely applicable methods for designing nonlinear systems, and is very widely used as a standard mathematical tool for analyzing limit cycles in closed-loop controllers, such as industrial process controls, servomechanisms, and electronic oscillators.
Nikolay Mitrofanovich Krylov (Russian: Никола́й Митрофа́нович Крыло́в , Ukrainian: Микола Митрофанович Крилов ) (29 November [O.S. 17 November] 1879 – May 11, 1955) was a Russian and Soviet mathematician known for works on interpolation, non-linear mechanics, and numerical methods for solving equations of mathematical physics.
Richard Lawrence Bishop (born 1932) is an American mathematician, a professor emeritus of mathematics at the University of Illinois at Urbana–Champaign.
 The Bishop–Gromov inequality in Riemannian geometry is named after him (with Mikhail Gromov).
Mikhail Leonidovich Lozinsky (Russian: Михаил Леонидович Лозинский, 20 July 1886 – 31 January 1955) is deemed to be the most accomplished Russian translator of the 20th century.
 "In the difficult and noble art of translation," said Anna Akhmatova, "Lozinsky was for the twentieth century what Zhukovsky was for the nineteenth."
 Lozinsky's granddaughters include writers Tatyana Tolstaya and Natalia Tolstaya.
Nicolai Vladimirovich Krylov (Russian: Никола́й Влади́мирович Крыло́в ; born 5 June 1941) is a Russian mathematician specializing in partial differential equations, particularly stochastic partial differential equations and diffusion processes.
 Krylov studied at Lomonosov University, where he in 1966 under E. B. Dynkin attained a doctoral candidate title (similar to a PhD) and in 1973 a Russian doctoral degree (somewhat more prestigious than a PhD).
 He taught from 1966 to 1990 at the Lomonosov University and is since 1990 a professor at the University of Minnesota.
 At the beginning of his career (starting from 1963) he, in collaboration with Dynkin, worked on nonlinear stochastic control theory, making advances in the study of convex, nonlinear partial equations of 2nd order ("i.e." Bellman equations), which were examined with stochastic methods.
 This led to the Evans-Krylov theory, for which he received with Lawrence C. Evans in 2004 the Leroy P. Steele Prize of the American Mathematical Society (for work done simultaneously and independently by both Krylov and Evans).
 They proved the second order differentiability (Hölder continuity of the second derivative) of the solutions of convex, completely nonlinear, second order elliptical partial differential equations and thus the existence of "classical solutions" (Theorem of Evans-Krylov).
 He was in 1978 at Helsinki and in 1986 at Berkeley an Invited Speaker for the ICM.
 He received the Humboldt Research Award in 2001.
 In 1993 he was elected a member of the American Academy of Arts and Sciences (1993).
 He should not be confused with the mathematician Nikolay M. Krylov.
Mikhail Leonidovich Gromov (also Mikhael Gromov, Michael Gromov or Mischa Gromov; Russian: Михаи́л Леони́дович Гро́мов ; born 23 December 1943), is a French-Russian mathematician known for work in geometry, analysis and group theory.
 He is a permanent member of IHÉS in France and a Professor of Mathematics at New York University.
In mathematics, the Gromov product is a concept in the theory of metric spaces named after the mathematician Mikhail Gromov.
 The Gromov product can also be used to define "δ"-hyperbolic metric spaces in the sense of Gromov.
Mikhail Leonidovich Slonimsky (Russian: Михаи́л Леони́дович Слони́мский ; 1 August [O.S. 20 July] 1897 – 8 October 1972) was a Soviet writer, member of the Serapion Brothers group.
In mathematics, the Krylov–Bogolyubov theorem (also known as the existence of invariant measures theorem) may refer to either of the two related fundamental theorems within the theory of dynamical systems.
 The theorems guarantee the existence of invariant measures for certain "nice" maps defined on "nice" spaces and were named after Russian-Ukrainian mathematicians and theoretical physicists Nikolay Krylov and Nikolay Bogolyubov who proved the theorems.
Open Problems in Mathematics is a book, edited by John Forbes Nash Jr. and Michael Th.
 Rassias, published in 2016 by Springer ().
 The book consists of seventeen expository articles, written by outstanding researchers, on some of the central open problems in the field of mathematics.
 The book also features an Introduction on "John Nash: Theorems and Ideas", by Mikhail Leonidovich Gromov.
 According to the editors’ Preface, each article is devoted to one open problem or a “constellation of related problems”.

Question: Which mathematician is known in part for his work on interpolation, Nikolay Mitrofanovich Krylov or Mikhail Leonidovich Gromov?
Answer: Nikolay Mitrofanovich Krylov