The erratic motion of pollen grains and other tiny particles suspended
in liquid is known as Brownian motion, after its discoverer, Robert
Brown, a botanist who worked in 1828, in London. He turned over the
problem of why this motion occurred to physicists who were investigating
kinetic theory and thermodynamics; at a time when the existence of
molecules had yet to be established. In 1900, Henri Poincare lectured on
this topic to the 1900 International Congress of Physicists, in Paris
[Wic95]. At this time, Louis Bachelier, a thesis student of Poincare,
made a monumental breakthrough with his Theory of Stock Market
Fluctuations, which is still studied today, [Co064]. Norbert Wiener
(1923), who was first to formulate a rigorous concept of the Brownian
path, is most often cited by mathematicians as the father of the
subject, while physicists will cite A. Einstein (1905) and M.
Smoluchowski. Both considered Markov diffusions and realized that
Brownian behaviour nd could be formulated in terms of parabolic 2 order
linear p. d. e. 'so Further- more, from this perspective, the covariance
of changes in position could be allowed to depend on the position
itself, according to the invariant form of the diffusion introduced by
Kolmogorov in 1937, [KoI37]. Thus, any time- homogeneous Markov
diffusion could be written in terms of the Laplacian, intrinsically
given by the symbol (covariance) of the p. d. e., plus a drift vec- tor.
The theory was further advanced in 1949, when K.