The gamma function, written as G(x), was first introduced by the mathematician Leonhard Euler (1707–1783) as a general form of the factorial function x! that could be applied to complex and negative numbers. Later, Adrien-Marie Legendre (1752–1833) who provided a “duplication formula” for the G function, introduced the notation that is commonly used now. Karl Weierstrass (1815–1897) expanded and developed the theory of G functions in the complex plane.