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