Jensen's formula
In the mathematical field known as complex analysis, Jensen's formula, introduced by , relates the average magnitude of an analytic function on a circle with the number of its zeros inside the circle. It forms an important statement in the study of entire functions. Formal statement Suppose that is an analytic function in a region in the complex plane which contains the closed disk of radius about the origin, are the zeros of in the interior of (repeated according to their respective multiplicity), and that . Jensen's formula states that This formula establishes a connection between the moduli of the zeros of the function inside the disk and the average of on the boundary circle , and can be seen as a generalisation of the mean value property of harmonic functions. Namely, if has no zeros in , then Jensen's formula reduces to which is the mean-value property of the harmonic function . An equivalent statement of Jensen's formula that is frequently used is where denotes the number of zeros of in the disc of radius centered at the origin. Proof Applications Jensen's formula can be used to estimate the number of zeros of an analytic function in a circle. Namely, if is a function analytic in a disk of radius centered at and if is bounded by on the boundary of that disk, then the number of zeros of in a circle of radius centered at the same point does not exceed Jensen's formula is an important statement in the study of value distribution of entire and meromorphic functions. In particular, it is the starting point of Nevanlinna theory, and it often appears in proofs of Hadamard factorization theorem, which requires an estimate on the number of zeros of an entire function. Generalizations Jensen's formula may be generalized for functions which are merely meromorphic on . Namely, assume that where and are analytic functions in having zeros at and respectively, then Jensen's formula for meromorphic functions states that Jensen's formula is a consequence of the more general Poisson–Jensen formula, which in turn follows from Jensen's formula by applying a Möbius transformation to . It was introduced and named by Rolf Nevanlinna. If is a function which is analytic in the unit disk, with zeros located in the interior of the unit disk, then for every in the unit disk the Poisson–Jensen formula states that Here, is the Poisson kernel on the unit disk. If the function has no zeros in the unit disk, the Poisson-Jensen formula reduces to which is the Poisson formula for the harmonic function . References Sources Theorems in complex analysis