Laguerre–Pólya class


The Laguerre–Pólya class is the class of entire functions consisting of those functions which are locally the limit of a series of polynomials whose roots are all real.
Any function of Laguerre–Pólya class is also of Pólya class.
The product of two functions in the class is also in the class, so the class constitutes a monoid under the operation of function multiplication.
Some properties of a function in the Laguerre–Pólya class are:
A function is of Laguerre–Pólya class if and only if three conditions are met:
with b and c real and c non-positive.

Examples

Some examples are
On the other hand, are not in the Laguerre–Pólya class.
For example,
Cosine can be done in more than one way. Here is one series of polynomials having all real roots:
And here is another:
This shows the buildup of the Hadamard product for cosine.
If we replace z2 with z, we have another function in the class:
Another example is the reciprocal gamma function 1/Γ. It is the limit of polynomials as follows: