The first person to describe the mathematics behind Brownian motion was Thorvald N. Thiele in a paper on the method of least squares published in 1880. This was followed by Louis Bachelier in 1900 in his PhD thesis "The Theory of Speculation", in which he presented an analysis of the stock and option markets. The Brownian motion model of the stock market is often used, but Benoit Mandelbrot denied its applicability to stock price movements.