James–Stein estimator: Difference between revisions

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In [[combinatorics|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 polynomial]]s.  See also [[Sheffer sequence]].
 
==Definition==
The q-difference polynomials satisfy the relation
 
:<math>\left(\frac {d}{dz}\right)_q p_n(z) =
\frac{p_n(qz)-p_n(z)} {qz-z} = p_{n-1}(z)</math>
 
where the derivative symbol on the left is the q-derivative. In the limit of <math>q\to 1</math>, this becomes the definition of the Appell polynomials:
 
:<math>\frac{d}{dz}p_n(z) = p_{n-1}(z).</math>
 
==Generating function==
The [[generating function]] for these polynomials is of the type of generating function for Brenke polynomials, namely
 
:<math>A(w)e_q(zw) = \sum_{n=0}^\infty p_n(z) w^n</math>
 
where <math>e_q(t)</math> is the [[q-exponential]]:
:<math>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}.</math>
 
Here, <math>[n]_q!</math> is the [[q-factorial]] and
 
:<math>(q;q)_n=(1-q^n)(1-q^{n-1})\cdots (1-q)</math>
 
is the [[q-Pochhammer symbol]]. The function <math>A(w)</math> is arbitrary but assumed to have an expansion
 
:<math>A(w)=\sum_{n=0}^\infty a_n w^n \mbox{ with } a_0 \ne 0. </math>
 
Any such <math>A(w)</math> 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.)''
 
[[Category:Q-analogs]]
[[Category:Polynomials]]

Latest revision as of 00:53, 17 December 2014

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