A type of random equation of motion (see stochastic process) used to study Brownian motion. The Langevin equation can be written in the form v̇=ξ‎v+A(t), where v is the velocity of a particle of mass m immersed in a fluid and v̇ is the acceleration of the particle; ξ‎v is a frictional force resulting from the viscosity of the fluid, with ξ‎ being a constant friction coefficient, and A(t) is a random force describing the average effect of the Brownian motion. The Langevin equation is named after the French physicist Paul Langevin (1872–1946), who introduced it in 1908. It is necessary to use statistical methods and the theory of probability to solve the Langevin equation.