Angelescu polynomials

In mathematics, the Angelescu polynomials π_n(x) are a series of polynomials generalizing the Laguerre polynomials introduced by (Angelescu 1938). The polynomials can be given by the generating function[math]\displaystyle{ \phi\left(\frac t{1-t}\right)\exp\left(-\frac{xt}{1-t}\right)=\sum_{n=0}^\infty\pi_n(x)t^n. }[/math](Boas Buck)

They can also be defined by the equation [math]\displaystyle{ \pi_{n}(x) := e^x D^n[e^{-x}A_n(x)], }[/math]where [math]\displaystyle{ \frac{A_n(x)}{n!} }[/math] is an Appell set of polynomials^[which?] (see (Shukla 1981)).

Properties

Addition and recurrence relations

The Angelescu polynomials satisfy the following addition theorem:

[math]\displaystyle{ (-1)^n\sum_{r=0}^m\frac{L_{m+n-r}^{(n)}(x)\pi_r(y)}{(n+m-r)!r!} = \sum_{r=0}^m (-1)^r\binom{-n-1}{r} \frac{\pi_{n-r}(x+y)}{(m-r)!}, }[/math]where [math]\displaystyle{ L^{(n)}_{m+n-r} }[/math] is a generalized Laguerre polynomial.

A particularly notable special case of this is when [math]\displaystyle{ n=0 }[/math], in which case the formula simplifies to[math]\displaystyle{ \frac{\pi_m(x+y)}{m!} = \sum_{r=0}^m \frac{L_{m-r}(x)\pi_r(y)}{(m-r)!r!} - \sum_{r=0}^{m-1} \frac{L_{m-r-1}(x)\pi_r(y)}{(m-r-1)!r!}. }[/math](Shastri 1940)^{[clarification needed]}

The polynomials also satisfy the recurrence relation

[math]\displaystyle{ \pi_s(x) = \sum_{r=0}^n (-1)^{n+r}\binom{n}{r}\frac{s!}{(n+s-r)!}\frac{d^n}{dx^n}[\pi_{n+s-r}(x)], }[/math]

which simplifies when [math]\displaystyle{ n=0 }[/math] to [math]\displaystyle{ \pi'_{s+1}(x) = (s+1)[\pi'_s(x) - \pi_s(x)] }[/math]. ((Shastri 1940)) This can be generalized to the following:

[math]\displaystyle{ -\sum_{r=0}^s \frac{1}{(m+n-r-1)!}L^{(m+n-1)}_{m+n-r-1}(x)\frac{\pi_{r-s}(y)}{(s-r)!} = \frac{1}{(m+n+s)!}\frac{d^{m+n}}{dx^m dy^n}\pi_{m+n+s}(x+y), }[/math]

a special case of which is the formula [math]\displaystyle{ \frac{d^{m+n}}{dx^m dy^n}\pi_{m+n}(x+y) = (-1)^{m+n} (m+n)! a_0 }[/math]. (Shastri 1940)

Integrals

The Angelescu polynomials satisfy the following integral formulae:

[math]\displaystyle{ \begin{align} \int_0^{\infty}\frac{e^{-x/2}}{x}[\pi_n(x) - \pi_n(0)]dx &= \sum_{r=0}^{n-1} (-1)^{n-r+1}\frac{n!}{r!}\pi_r(0)\int_0^{\infty} [\frac{1}{1/2 + p} - 1]^{n-r-1} d[\frac{1}{1/2+p}]\\ &= \sum_{r=0}^{n-1} (-1)^{n-r+1}\frac{n!}{r!}\frac{\pi_r(0)}{n-r}[1 + (-1)^{n-r-1}] \end{align} }[/math]

[math]\displaystyle{ \int_0^{\infty} e^{-x}[\pi_n(x) - \pi_n(0)]L_m^{(1)}(x)dx = \begin{cases} 0\text{ if }m\geq n\\ \frac{n!}{(n-m-1)!}\pi_{n-m-1}(0)\text{ if }0\leq m\leq n-1 \end{cases} }[/math]

(Shastri 1940)

(Here, [math]\displaystyle{ L_m^{(1)}(x) }[/math] is a Laguerre polynomial.)

Further generalization

We can define a q-analog of the Angelescu polynomials as [math]\displaystyle{ \pi_{n, q}(x) := e_q(xq^n) D_q^n[E_q(-x)P_n(x)] }[/math], where [math]\displaystyle{ e_q }[/math] and [math]\displaystyle{ E_q }[/math] are the q-exponential functions [math]\displaystyle{ e_q(x) := \Pi_{n=0}^{\infty} (1 - q^n x)^{-1} = \Sigma_{k=0}^{\infty}\frac{x^k}{[k]!} }[/math] and [math]\displaystyle{ E_q(x) := \Pi_{n=0}^{\infty} (1 + q^n x) = \Sigma_{k=0}^{\infty}\frac{q^{\frac{k(k-1)}{2}}x^k}{[k]!} }[/math]^{[verification needed]}, [math]\displaystyle{ D_q }[/math] is the q-derivative, and [math]\displaystyle{ P_n }[/math] is a "q-Appell set" (satisfying the property [math]\displaystyle{ D_q P_n(x) = [n]P_{n-1}(x) }[/math]). (Shukla 1981)

This q-analog can also be given as a generating function as well:

[math]\displaystyle{ \sum_{n=0}^{\infty}\frac{\pi_{n, q}(x)t^n}{(1;n)} = \sum_{n=0}^{\infty}\frac{(-1)^n q^{\frac{n(n-1)}{2}}t^n P_n(x)}{(1;n)[1-t]_{n+1}}, }[/math]where we employ the notation [math]\displaystyle{ (a;k) := (1 - q^a)\dots (1 - q^{a+k-1}) }[/math] and [math]\displaystyle{ [a+b]_n = \sum_{k=0}^n\begin{bmatrix}n\\k\end{bmatrix}a^{n-k}b^k }[/math]. (Shukla 1981)^{[verification needed]}

References

• Angelescu, A. (1938), "Sur certains polynomes généralisant les polynomes de Laguerre." (in French), C. R. Acad. Sci. Roumanie 2: 199–201 
• Boas, Ralph P.; Buck, R. Creighton (1958), Polynomial expansions of analytic functions, Ergebnisse der Mathematik und ihrer Grenzgebiete. Neue Folge., 19, Berlin, New York: Springer-Verlag, ISBN 9783540031239, https://books.google.com/books?id=eihMuwkh4DsC 
• Shukla, D. P. (1981). "q-Angelescu polynomials". Publications de l'Institut Mathématique 43: 205–213. http://elib.mi.sanu.ac.rs/files/journals/publ/49/n043p205.pdf. 
• Shastri, N. A. (1940). "On Angelescu's polynomial πn (x)". Proceedings of the Indian Academy of Sciences, Section A 11 (4): 312–317. doi:10.1007/BF03051347. https://www.ias.ac.in/article/fulltext/seca/011/04/0312-0317. 

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