Johann Peter Gustav Lejeune Dirichlet (German: [ləˈʒœn diʀiˈkleː]; 13 February 1805 – 5 May 1859) was a German mathematician who made deep contributions to number theory (including creating the field of analytic number theory), and to the theory of Fourier series and other topics in mathematical analysis; he is credited with being one of the first mathematicians to give the modern formal definition of a function. Although his surname is Lejeune Dirichlet, he is commonly referred to as just Dirichlet, in particular for results named after him.