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.
Valery Denisovich Goppa (Russian: Вале́рий Дени́сович Го́ппа ; born 1939) is a Soviet and Russian mathematician.
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.
John B. Bell is an American mathematician and the head of the Center for Computational Sciences and Engineering at the Lawrence Berkeley National Laboratory.
 He has made contributions in the areas of finite difference methods, numerical methods for low Mach number flows, adaptive mesh refinement, interface tracking and parallel computing.
 He has also worked on the application of these numerical methods to problems from a broad range of fields, including combustion, shock physics, seismology, flow in porous media and astrophysics.
Fluid mechanics is a branch of physics concerned with the mechanics of fluids (liquids, gases, and plasmas) and the forces on them.
 Fluid mechanics has a wide range of applications, including mechanical engineering, civil engineering, chemical engineering, biomedical engineering, geophysics, astrophysics, and biology.
 Fluid mechanics can be divided into fluid statics, the study of fluids at rest; and fluid dynamics, the study of the effect of forces on fluid motion.
 It is a branch of continuum mechanics, a subject which models matter without using the information that it is made out of atoms; that is, it models matter from a "macroscopic" viewpoint rather than from "microscopic".
 Fluid mechanics, especially fluid dynamics, is an active field of research with many problems that are partly or wholly unsolved.
 Fluid mechanics can be mathematically complex, and can best be solved by numerical methods, typically using computers.
 A modern discipline, called computational fluid dynamics (CFD), is devoted to this approach to solving fluid mechanics problems.
 Particle image velocimetry, an experimental method for visualizing and analyzing fluid flow, also takes advantage of the highly visual nature of fluid flow.
General linear methods (GLMs) are a large class of numerical methods used to obtain numerical solutions to differential equations.
 This large class of methods in numerical analysis encompass multistage Runge–Kutta methods that use intermediate collocation points, as well as linear multistep methods that save a finite time history of the solution.
 John C. Butcher originally coined this term for these methods, and has written a series of review papers
The Krylov–Bogolyubov averaging method (Krylov–Bogolyubov method of averaging) is a mathematical method for approximate analysis of oscillating processes in non-linear mechanics.
 The method is based on the averaging principle when the exact differential equation of the motion is replaced by its averaged version.
 The method is named after Nikolay Krylov and Nikolay Bogoliubov.
Newton–Krylov methods are numerical methods for solving non-linear problems using Krylov subspace linear solvers.
Bertil Gustafsson (born 1939) is a Swedish applied mathematician and numerical analyst.
 He is currently a Professor emeritus in the Department of Information Technology at Uppsala University, Sweden.
 Gustafsson is known for his work in numerical methods for time-dependent partial differential equations and its applications in fluid dynamics.
 He is the G in GKS (Gustafsson–Kreiss–Sundstrom) theory for initial-boundary value problems which discusses the stability criterion for numerical approximations of initial–boundary value problems.
 Gustafsson has also authored a couple of books on the topic numerical methods applied to PDE.
Weakened weak form (or W2 form) is used in the formulation of general numerical methods based on meshfree methods and/or finite element method settings.
 These numerical methods are applicable to solid mechanics as well as fluid dynamics problems.

Question: In between Nikolay Mitrofanovich Krylov and Valery Goppa who is known for works on interpolation, non-linear mechanics, and numerical methods?
Answer: Nikolay Mitrofanovich Krylov