Johann Peter Gustav Lejeune Dirichlet (; 13 February 1805 – 5 May 1859) was a German mathematician credited with the modern formal definition of a function. Biography His family was from Richelette, a small community 5 km north east of Liège in Belgium, from which his surname "Lejeune Dirichlet" ("", French for "the youth from Richelette") was derived. Dirichlet was born in Düren, where his father was the postmaster. He learned from Georg Ohm at the Jesuit gymnasium in Cologne… (More on Dirichlet)