One of the forms of the Lindelöf principle, which employs the concept of subordination of functions. Let be any function regular in the disc and satisfying the conditions , in ; let be a meromorphic function in . If the function has the form , then is called subordinate to the function in the disc , while is called the subordinating function. This subordination relation is denoted by . In the simplest case where is a univalent function in , this relation simply means that and that does not take any values in the disc that are not taken there by . The following basic theorems apply.
Let the function be meromorphic in the disc and map it on the Riemann surface . Let be the part of corresponding to , . If , then the values of in (under analytic continuation from ) lie in , and the boundary points in are obtained only for , .
If and if is regular in , , then setting
one has , , , .
If and is regular at , then for the coefficients of the expansions , one has , .
The general theory of subordination is useful in considering the set of values taken or produced by an analytic function. The subordination relation can be used in two different ways. First, one can start from a given function and examine the behaviour of all subordinate to . If is completely known, then the region in which the values of lie is also known (Theorem 1) as well as an upper bound on the integral means (Theorem 2). If also is regular at , there are upper bounds for the coefficients of (Theorem 3). Secondly, one can consider a function that is meromorphic in the disc and whose properties imply that it cannot be subordinate to a given function in . If here , for example, is univalent, then necessarily takes values outside in . These ideas of using the subordination relation illustrate the subordination principle and can be extended to multiply-connected domains .
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Subordination principle. Yu.E. Alenitsyn (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Subordination_principle&oldid=18946