In mathematics, an automorphic factor is a certain type of analytic function, defined on subgroups of SL(2,R), appearing in the theory of modular forms. The general case, for general groups, is reviewed in the article 'factor of automorphy'. Definition An automorphic factor of weight k is a function satisfying the four properties given below. Here, the notation and refer to the upper half-plane and the complex plane, respectively. The notation is a subgroup of SL(2,R), such as, for example, a Fuchsian group. An element is a 2×2 matrix with a, b, c, d real numbers, satisfying ad−bc=1. An automorphic factor must satisfy: For a fixed , the function is a holomorphic function of . For all and , one has for a fixed real number k. For all and , one has Here, is the fractional linear transform of by . If , then for all and , one has Here, I denotes the identity matrix. Properties Every automorphic factor may be written as with The function is called a multiplier system. Clearly, , while, if , then which equals when k is an integer. References Robert Rankin, Modular Forms and Functions, (1977) Cambridge University Press . (Chapter 3 is entirely devoted to automorphic factors for the modular group.) Modular forms