Neumann polynomial

In mathematics, the Neumann polynomials, introduced by Carl Neumann for the special case \alpha=0, are a sequence of polynomials in 1/t used to expand functions in term of Bessel functions.Abramowitz and Stegun, [https://www.math.sfu.ca/~cbm/aands/page_363.htm p. 363, 9.1.82] ff.

The first few polynomials are

:O_0^{(\alpha)}(t)=\frac 1 t,

:O_1^{(\alpha)}(t)=2\frac {\alpha+1}{t^2},

:O_2^{(\alpha)}(t)=\frac {2+\alpha}{t}+ 4\frac {(2+\alpha)(1+\alpha)}{t^3},

:O_3^{(\alpha)}(t)=2\frac {(1+\alpha)(3+\alpha)}{t^2}+ 8\frac {(1+\alpha)(2+\alpha)(3+\alpha)}{t^4},

:O_4^{(\alpha)}(t)=\frac {(1+\alpha)(4+\alpha)}{2t}+ 4\frac {(1+\alpha)(2+\alpha)(4+\alpha)}{t^3}+ 16\frac {(1+\alpha)(2+\alpha)(3+\alpha)(4+\alpha)}{t^5}.

A general form for the polynomial is

:O_n^{(\alpha)}(t)= \frac{\alpha+n}{2\alpha} \sum_{k=0}^{\lfloor n/2\rfloor} (-1)^{n-k}\frac {(n-k)!} {k!} {-\alpha \choose n-k}\left(\frac 2 t \right)^{n+1-2k},

and they have the "generating function"

:\frac{\left(\frac z 2 \right)^\alpha} {\Gamma(\alpha+1)} \frac 1 {t-z}= \sum_{n=0}O_n^{(\alpha)}(t) J_{\alpha+n}(z),

where J are Bessel functions.

To expand a function f in the form

:f(z)=\left(\frac{2}{z}\right)^\alpha \sum_{n=0} a_n J_{\alpha+n}(z)\,

for |t|, compute

:a_n=\frac {\Gamma(\alpha+1)} {2 \pi i} \oint_{|t|=c'} f(t) O_n^{(\alpha)}(t)\,dt,

where c' and c is the distance of the nearest singularity of f(z) from z=0 .

Examples

An example is the extension

:\left(\tfrac{1}{2}z\right)^s= \Gamma(s)\cdot\sum_{k=0}(-1)^k J_{s+2k}(z)(s+2k){-s \choose k},

or the more general Sonine formula{{citation

| last1 = Erdélyi | first1 = Arthur

| last2 = Magnus | first2 = Wilhelm

| last3 = Oberhettinger | first3 = Fritz

| last4 = Tricomi | first4 = Francesco G.

| mr = 58756

| publisher = McGraw-Hill

| title = Higher Transcendental Functions. Vols. I, II, III

| year = 1955}} II.7.10.1, p.64

:e^{i \gamma z}= \Gamma(s)\cdot\sum_{k=0}i^k C_k^{(s)}(\gamma)(s+k)\frac{J_{s+k}(z)}{\left(\frac z 2\right)^s}.

where C_k^{(s)} is Gegenbauer's polynomial. Then,{{fact|date=September 2011}}{{or|date=September 2011}}

:\frac{\left(\frac z 2\right)^{2k}}{(2k-1)!}J_s(z)= \sum_{i=k}(-1)^{i-k}{i+k-1\choose 2k-1}{i+k+s-1\choose 2k-1}(s+2i)J_{s+2i}(z),

:\sum_{n=0} t^n J_{s+n}(z)= \frac{e^{\frac{t z}2}}{t^s} \sum_{j=0}\frac{\left(-\frac{z}{2t}\right)^j}{j!}\frac{\gamma \left(j+s,\frac{t z}{2}\right)}{\,\Gamma (j+s)}= \int_0^\infty e^{-\frac{z x^2}{2 t}}\frac {z x}{t} \frac{J_s(z\sqrt{1-x^2})}{\sqrt{1-x^2}^s}\,dx,

the confluent hypergeometric function

:M(a,s,z)= \Gamma (s) \sum_{k=0}^\infty \left(-\frac{1}{t}\right)^k L_k^{(-a-k)}(t) \frac{J_{s+k-1}\left(2 \sqrt{t z}\right)}{(\sqrt{t z})^{s-k-1}},

and in particular

:\frac{J_s(2 z)}{z^s}= \frac{4^s \Gamma\left(s+\frac12\right)}{\sqrt\pi}e^{2 i z}\sum_{k=0}L_k^{(-s-1/2-k)}\left(\frac{it}4\right)(4 i z)^k \frac{J_{2s+k}\left(2\sqrt{t z}\right)}{\sqrt{t z}^{2s+k}},

the index shift formula

:\Gamma(\nu-\mu) J_\nu(z)= \Gamma(\mu+1) \sum_{n=0}\frac{\Gamma(\nu-\mu+n)}{n!\Gamma(\nu+n+1)} \left(\frac z 2\right)^{\nu-\mu+n}J_{\mu+n}(z),

the Taylor expansion (addition formula)

:\frac{J_s\left(\sqrt{z^2-2uz}\right)}{\left(\sqrt{z^2-2uz}\right)^{\pm s}}= \sum_{k=0}\frac{(\pm u)^k}{k!}\frac{J_{s\pm k}(z)}{z^{\pm s}},

(cf.{{cite book |author-first1=Izrail Solomonovich |author-last1=Gradshteyn |author-link1=Izrail Solomonovich Gradshteyn |author-first2=Iosif Moiseevich |author-last2=Ryzhik |author-link2=Iosif Moiseevich Ryzhik |author-first3=Yuri Veniaminovich |author-last3=Geronimus |author-link3=Yuri Veniaminovich Geronimus |author-first4=Michail Yulyevich |author-last4=Tseytlin |author-link4=Michail Yulyevich Tseytlin |author-first5=Alan |author-last5=Jeffrey |editor-first1=Daniel |editor-last1=Zwillinger |editor-first2=Victor Hugo |editor-last2=Moll |editor-link2=Victor Hugo Moll |translator=Scripta Technica, Inc. |title=Table of Integrals, Series, and Products |publisher=Academic Press, Inc. |date=2015 |orig-year=October 2014 |edition=8 |language=English |isbn=0-12-384933-0 |lccn=2014010276 |title-link=Gradshteyn and Ryzhik |chapter=8.515.1. |page=944}}{{verification failed|date=September 2011|reason=The referenced identity seems to be only superficially similar. It does not directly support the identity here.}}) and the expansion of the integral of the Bessel function,

:\int J_s(z)dz= 2 \sum_{k=0} J_{s+2k+1}(z),

are of the same type.

See also

Notes