Difference polynomials
# Difference polynomials

In mathematics, in the area of complex analysis, the general difference polynomials are a polynomial sequence, a certain subclass of the Sheffer polynomials, which include the Newton polynomials, Selberg's polynomials, and the Stirling interpolation polynomials as special cases.

## Definition

The general difference polynomial sequence is given by

    $p_{n}(z)={\frac {z}{n}}{{z-\beta n-1} \choose {n-1}}$

where ${z \choose n}$ is the binomial coefficient. For $\beta =0$, the generated polynomials $p_{n}(z)$ are the Newton polynomials

    $p_{n}(z)={z \choose n}={\frac {z(z-1)\cdots (z-n+1)}{n!}}.$

The case of $\beta =1$ generates Selberg's polynomials, and the case of $\beta =-1/2$ generates Stirling's interpolation polynomials.

## Moving differences

Given an analytic function $f(z)$, define the moving difference of f as

    ${\mathcal {L}}_{n}(f)=\Delta ^{n}f(\beta n)$

where $\Delta$ is the forward difference operator. Then, provided that f obeys certain summability conditions, then it may be represented in terms of these polynomials as

    $f(z)=\sum _{n=0}^{\infty }p_{n}(z){\mathcal {L}}_{n}(f).$

The conditions for summability (that is, convergence) for this sequence is a fairly complex topic; in general, one may say that a necessary condition is that the analytic function be of less than exponential type. Summability conditions are discussed in detail in Boas & Buck.

## Generating function

The generating function for the general difference polynomials is given by

    $e^{zt}=\sum _{n=0}^{\infty }p_{n}(z)\left[\left(e^{t}-1\right)e^{\beta t}\right]^{n}.$

This generating function can be brought into the form of the generalized Appell representation

    $K(z,w)=A(w)\Psi (zg(w))=\sum _{n=0}^{\infty }p_{n}(z)w^{n}$

by setting $A(w)=1$, $\Psi (x)=e^{x}$, $g(w)=t$ and $w=(e^{t}-1)e^{\beta t}$.

