In complex analysis, a branch of mathematics, a holomorphic function is said to be of exponential type C if its growth is bounded by the exponential function eC|z| for some constant C as |z|→∞. When a function is bounded in this way, it is then possible to express it as certain kinds of convergent summations over a series of other complex functions, as well as understanding when it is possible to apply techniques such as Borel summation, or, for example, to apply the Mellin transform, or to perform approximations using the Euler-MacLaurin formula. The general case is handled by Nachbin's theorem, which defines the analogous notion of Ψ-type for a general function Ψ(z) as opposed to ez.
A function f(z) defined on the complex plane is said to be of exponential type if there exist constants M and τ such that
in the limit of . Here, the complex variable z was written as to emphasize that the limit must hold in all directions θ. Letting τ stand for the infimum of all such τ, one then says that the function f is of exponential type τ.
For example, let . Then one says that is of exponential type π, since π is the smallest number that bounds the growth of along the imaginary axis. So, for this example, Carlson's theorem cannot apply, as it requires functions of exponential type less than π. Similarly, the Euler-MacLaurin formula cannot be applied either, as it, too, expresses an theorem ultimately anchored in the theory of finite differences.
A holomorphic function function is said to be of exponential type if for every there exists a constant such that
for where . We say is of exponential type if is of exponential type for some . The number
is the exponential type of .
Exponential type with respect to a symmetric convex body
Stein (1957) has given a generalization of exponential type for entire functions of several complex variables. Suppose is a convex, compact, and symmetric subset of . It is known that for every such there is an associated norm with the property that
In other words, is the unit ball in with respect to . The set
We extend from to by
An entire function of -complex variables is said to be of exponential type with respect to if for every there exists a constant such that
for all .