In mathematics, in the field of complex analysis, a Nevanlinna function is a complex function which is an analytic function on the open upper half-plane and has a non-negative imaginary part. A Nevanlinna function maps the upper half-plane to itself or a real constant,A real number is not considered to be in the upper half-plane. but is not necessarily injective or surjective. Functions with this property are sometimes also known as Herglotz, Pick or R functions.
Integral representation
Every Nevanlinna function admits a representation
where is a real constant, is a non-negative constant, is the upper half-plane, and is a Borel measure on satisfying the growth condition
Conversely, every function of this form turns out to be a Nevanlinna function. The constants in this representation are related to the function via
and the Borel measure can be recovered from by employing the Stieltjes inversion formula (related to the inversion formula for the Stieltjes transformation):
A very similar representation of functions is also called the Poisson representation.See for example Section 4, "Poisson representation" in De Branges gives a form for functions whose real part is non-negative in the upper half-plane.
Examples Some elementary examples of Nevanlinna functions follow (with appropriately chosen branch cuts in the first three). ( can be replaced by for any real number .)
These are injective but when does not equal 1 or −1 they are not surjective and can be rotated to some extent around the origin, such as .
A sheet of such as the one with .
(an example that is surjective but not injective).
A Möbius transformation
is a Nevanlinna function if (sufficient but not necessary) is a positive real number and . This is equivalent to the set of such transformations that map the real axis to itself. One may then add any constant in the upper half-plane, and move the pole into the lower half-plane, giving new values for the parameters. Example:
and are examples which are entire functions. The second is neither injective nor surjective. If is a self-adjoint operator in a Hilbert space and is an arbitrary vector, then the function
is a Nevanlinna function.
If and are both Nevanlinna functions, then the composition is a Nevanlinna function as well.
Importance in operator theory
Nevanlinna functions appear in the study of Operator monotone functions.
References
General