Appell sequence

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In mathematics, an Appell sequence, named after Paul Émile Appell, is any polynomial sequence satisfying the identity

and in which is a non-zero constant.

Among the most notable Appell sequences besides the trivial example are the Hermite polynomials, the Bernoulli polynomials, and the Euler polynomials. Every Appell sequence is a Sheffer sequence, but most Sheffer sequences are not Appell sequences. Appell sequences have a probabilistic interpretation as systems of moments.

Equivalent characterizations of Appell sequences

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The following conditions on polynomial sequences can easily be seen to be equivalent:

  • For  ,
 
and   is a non-zero constant;
  • For some sequence   of scalars with  ,
 
  • For the same sequence of scalars,
 
where
 
  • For  ,
 

Recursion formula

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Suppose

 

where the last equality is taken to define the linear operator   on the space of polynomials in  . Let

 

be the inverse operator, the coefficients   being those of the usual reciprocal of a formal power series, so that

 

In the conventions of the umbral calculus, one often treats this formal power series   as representing the Appell sequence  . One can define

 

by using the usual power series expansion of the   and the usual definition of composition of formal power series. Then we have

 

(This formal differentiation of a power series in the differential operator   is an instance of Pincherle differentiation.)

In the case of Hermite polynomials, this reduces to the conventional recursion formula for that sequence.

Subgroup of the Sheffer polynomials

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The set of all Appell sequences is closed under the operation of umbral composition of polynomial sequences, defined as follows. Suppose   and   are polynomial sequences, given by

 

Then the umbral composition   is the polynomial sequence whose  th term is

 

(the subscript   appears in  , since this is the  th term of that sequence, but not in  , since this refers to the sequence as a whole rather than one of its terms).

Under this operation, the set of all Sheffer sequences is a non-abelian group, but the set of all Appell sequences is an abelian subgroup. That it is abelian can be seen by considering the fact that every Appell sequence is of the form

 

and that umbral composition of Appell sequences corresponds to multiplication of these formal power series in the operator  .

Different convention

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Another convention followed by some authors (see Chihara) defines this concept in a different way, conflicting with Appell's original definition, by using the identity

 

instead.

Hypergeometric Appell polynomials

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The enormous class of Appell polynomials can be obtained in terms of the generalized hypergeometric function.

Let   denote the array of   ratios

 

Consider the polynomial  

where   is the generalized hypergeometric function.

Theorem. The polynomial family   is the Appell sequence for any natural parameters  .

For example, if       then the polynomials   become the Gould-Hopper polynomials   and if   they become the Hermite polynomials  .

See also

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References

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  • Appell, Paul (1880). "Sur une classe de polynômes". Annales Scientifiques de l'École Normale Supérieure. 2e Série. 9: 119–144. doi:10.24033/asens.186.
  • Roman, Steven; Rota, Gian-Carlo (1978). "The Umbral Calculus". Advances in Mathematics. 27 (2): 95–188. doi:10.1016/0001-8708(78)90087-7..
  • Rota, Gian-Carlo; Kahaner, D.; Odlyzko, Andrew (1973). "Finite Operator Calculus". Journal of Mathematical Analysis and Applications. 42 (3): 685–760. doi:10.1016/0022-247X(73)90172-8. Reprinted in the book with the same title, Academic Press, New York, 1975.
  • Steven Roman. The Umbral Calculus. Dover Publications.
  • Theodore Seio Chihara (1978). An Introduction to Orthogonal Polynomials. Gordon and Breach, New York. ISBN 978-0-677-04150-6.
  • Bedratyuk, L.; Luno, N. (2020). "Some Properties of Generalized Hypergeometric Appell Polynomials". Carpathian Math. Publ. 12 (1): 129–137. arXiv:2005.01676. doi:10.15330/cmp.12.1.129-137.
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