James–Stein estimator

Template:Lowercase In combinatorial mathematics, the q-difference polynomials or q-harmonic polynomials are a polynomial sequence defined in terms of the q-derivative. They are a type of Brenke polynomial, and generalize the Appell polynomials. See also Sheffer sequence.

Definition

The q-difference polynomials satisfy the relation

$\left({\frac {d}{dz}}\right)_{q}p_{n}(z)={\frac {p_{n}(qz)-p_{n}(z)}{qz-z}}=p_{n-1}(z)$

where the derivative symbol on the left is the q-derivative. In the limit of $q\to 1$, this becomes the definition of the Appell polynomials:

${\frac {d}{dz}}p_{n}(z)=p_{n-1}(z).$

Generating function

The generating function for these polynomials is of the type of generating function for Brenke polynomials, namely

$A(w)e_{q}(zw)=\sum _{n=0}^{\infty }p_{n}(z)w^{n}$

where $e_{q}(t)$ is the q-exponential:

$e_{q}(t)=\sum _{n=0}^{\infty }{\frac {t^{n}}{[n]_{q}!}}=\sum _{n=0}^{\infty }{\frac {t^{n}(1-q)^{n}}{(q;q)_{n}}}.$

Here, $[n]_{q}!$ is the q-factorial and

$(q;q)_{n}=(1-q^{n})(1-q^{n-1})\cdots (1-q)$

is the q-Pochhammer symbol. The function $A(w)$ is arbitrary but assumed to have an expansion

$A(w)=\sum _{n=0}^{\infty }a_{n}w^{n}{\mbox{ with }}a_{0}\neq 0.$

Any such $A(w)$ gives a sequence of q-difference polynomials.

References

• A. Sharma and A. M. Chak, "The basic analogue of a class of polynomials", Riv. Mat. Univ. Parma, 5 (1954) 325-337.
• Ralph P. Boas, Jr. and R. Creighton Buck, Polynomial Expansions of Analytic Functions (Second Printing Corrected), (1964) Academic Press Inc., Publishers New York, Springer-Verlag, Berlin. Library of Congress Card Number 63-23263. (Provides a very brief discussion of convergence.)