William Feller, born Vilibald Srećko Feller, was a Croatian-American mathematician and one of the greatest probabilists of the twentieth century. His book in two volumes, An Introduction to Probability Theory and its Applications, is still celebrated as a classic and monumental work. Persi Diaconis (among other things, the man who proved that seven shuffles suffice to randomize a deck of cards) decided to go college after being a magician on the road for years because, in part, he wanted to be able to understand the mathematics in Feller’s book (source).

In the words of Mark Kac, “Feller was a man of enormous vitality.”