Jakobijevi polinomi, često zvani i hipergeometrijski polinomi su klasični ortogonalni polinom predstavljeni formulom:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(z)={\frac {\Gamma (\alpha +n+1)}{n!\,\Gamma (\alpha +\beta +n+1)}}\sum _{m=0}^{n}{n \choose m}{\frac {\Gamma (\alpha +\beta +n+m+1)}{\Gamma (\alpha +m+1)}}\left({\frac {z-1}{2}}\right)^{m}~.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/cb72ba6ff8911f3a9abcf06af7a42148ddd33dfb)
Gegenbauerovi polinomi, Ležandrovi polinomi i Čebiševljevi polinomi predstavljaju specijalni slučaj Jakobijevih polinoma. Jakobijeve polinome otkrio je 1859. nemački matematičar Karl Gustav Jakobi.
Jakobijevi polinomi predstavljaju rešenje linerane homogene diferencijalne jednačine drugoga reda:
![{\displaystyle (1-x^{2})y''+(\beta -\alpha -(\alpha +\beta +2)x)y'+n(n+\alpha +\beta +1)y=0.\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/81284644380f0be4e43ce3c3fb005f92000d9dad)
Jakobijevi polinomi definisani su pomoću hipergeometrijske funkcije:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(z)={\frac {(\alpha +1)_{n}}{n!}}\,_{2}F_{1}\left(-n,1+\alpha +\beta +n;\alpha +1;{\frac {1-z}{2}}\right),}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c0e9e51a17de3666a18693c705746a76063a6581)
gde
predstavlja Pohhamerov simbol. U tom slučaju razvojem se dobija:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(z)={\frac {\Gamma (\alpha +n+1)}{n!\,\Gamma (\alpha +\beta +n+1)}}\sum _{m=0}^{n}{n \choose m}{\frac {\Gamma (\alpha +\beta +n+m+1)}{\Gamma (\alpha +m+1)}}\left({\frac {z-1}{2}}\right)^{m}~.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/cb72ba6ff8911f3a9abcf06af7a42148ddd33dfb)
Jakobijevi polinomi mogu da se definišu i pomoću Rodrigezove formule:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(z)={\frac {(-1)^{n}}{2^{n}n!}}(1-z)^{-\alpha }(1+z)^{-\beta }{\frac {d^{n}}{dz^{n}}}\left\{(1-z)^{\alpha }(1+z)^{\beta }(1-z^{2})^{n}\right\}~.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2e9b7f7a8a445d5bc2f2608b99b849847ad8cfa1)
Generirajuća funkcija Jakobijevih polinoma je:
![{\displaystyle \sum _{n=0}^{\infty }P_{n}^{(\alpha ,\beta )}(z)w^{n}=2^{\alpha +\beta }R^{-1}(1-w+R)^{-\alpha }(1+w+R)^{-\beta }~,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8726d54ca618c016ca3ff26f397ac8a6309ee0c1)
gde
![{\displaystyle R=R(z,w)={\big (}1-2zw+w^{2}{\big )}^{1/2}~,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/04de531d846352060112de494944fc6cd9d84b5e)
Relacije rekurzije za Jakobijeve polinome su:
![{\displaystyle {\begin{aligned}&2n(n+\alpha +\beta )(2n+\alpha +\beta -2)P_{n}^{(\alpha ,\beta )}(z)\\&\qquad =(2n+\alpha +\beta -1){\Big \{}(2n+\alpha +\beta )(2n+\alpha +\beta -2)z+\alpha ^{2}-\beta ^{2}{\Big \}}P_{n-1}^{(\alpha ,\beta )}(z)\\&\qquad \qquad -2(n+\alpha -1)(n+\beta -1)(2n+\alpha +\beta )P_{n-2}^{(\alpha ,\beta )}(z)~,\quad n=2,3,\dots \end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/192dfe04e8a6606660166b1c8db2603adc0390a5)
Nekoliko prvih polinoma je:
![{\displaystyle P_{0}^{(\alpha ,\beta )}(z)=1}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8ab1be0010d74fdc47d7429f18dbbf8063925f6c)
![{\displaystyle P_{1}^{(\alpha ,\beta )}(z)={\frac {1}{2}}\left[2(\alpha +1)+(\alpha +\beta +2)(z-1)\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0a95e6198603c0eb110abde34809dcd787796025)
![{\displaystyle P_{2}^{(\alpha ,\beta )}(z)={\frac {1}{8}}\left[4(\alpha +1)(\alpha +2)+4(\alpha +\beta +3)(\alpha +2)(z-1)+(\alpha +\beta +3)(\alpha +\beta +4)(z-1)^{2}\right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/44cf56154dd63ffe05a063e6733b2e8100cad972)
Za realno x Jakobijevi polinomi mogu da se pišu i kao:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(x)=\sum _{s}{n+\alpha \choose s}{n+\beta \choose n-s}\left({\frac {x-1}{2}}\right)^{n-s}\left({\frac {x+1}{2}}\right)^{s}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/cc513a6251b0d437f9b52da6134b2bfa2030fd9f)
gde su s ≥ 0 i n-s ≥ 0, a za celobrojno n
![{\displaystyle {z \choose n}={\frac {\Gamma (z+1)}{\Gamma (n+1)\Gamma (z-n+1)}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d6668e8f4ff4bf4162d399a1ec429a3ab32ee40e)
U gornjoj jednačini Γ(z) je gama funkcija.
U specijalnom slučaju, kada su n, n+α, n+β, and
n+α+β nenegativni celi brojevi Jakobijevi polinomi mogu da se napišu kao:
![{\displaystyle {\begin{aligned}&P_{n}^{(\alpha ,\beta )}(x)=(n+\alpha )!(n+\beta )!\\&\qquad \times \sum _{s}\left[s!(n+\alpha -s)!(\beta +s)!(n-s)!\right]^{-1}\left({\frac {x-1}{2}}\right)^{n-s}\left({\frac {x+1}{2}}\right)^{s}.\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7bf8b0987f766a9d3fbfeedf257f60747fc98421)
Jakobijevi polinomi za α > -1 i β > -1 zadovoljavaju uslov ortogonalnosti:
![{\displaystyle {\begin{aligned}&\int _{-1}^{1}(1-x)^{\alpha }(1+x)^{\beta }P_{m}^{(\alpha ,\beta )}(x)P_{n}^{(\alpha ,\beta )}(x)\;dx\\&\quad ={\frac {2^{\alpha +\beta +1}}{2n+\alpha +\beta +1}}{\frac {\Gamma (n+\alpha +1)\Gamma (n+\beta +1)}{\Gamma (n+\alpha +\beta +1)n!}}\delta _{nm}\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/aac3d4ef2824399f88c3ffc1635b6be3b14f9fe5)
Težinska funkcija je bila:
.
Oni nisu ortonormalni, a za normalizaciju:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(1)={n+\alpha \choose n}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5ea914c1a25729ba439473ae730691436177d86e)
Jakobijevi polinomi zadovoljavaju sledeće relacije simetrije:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(-z)=(-1)^{n}P_{n}^{(\beta ,\alpha )}(z);}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2600d9ed56e859d5a202e9e439a6cbf4512bfd0a)
pa je
![{\displaystyle P_{n}^{(\alpha ,\beta )}(-1)=(-1)^{n}{n+\beta \choose n}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c74d400c2b4a877084a6640c2d549661e3916a0a)
Za x unutar intervala [-1, 1], asimptotska vrednost Pn(α,β) za veliki n dan je:
![{\displaystyle P_{n}^{(\alpha ,\beta )}(\cos \theta )=n^{-1/2}\cos(N\theta +\gamma )+O(n^{-3/2})~,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/40dee81dc07753e9c9e5d2f5834ace0a11c21f7c)
gde
![{\displaystyle {\begin{aligned}k(\theta )&=\pi ^{-1/2}\sin ^{-\alpha -1/2}{\frac {\theta }{2}}\cos ^{-\beta -1/2}{\frac {\theta }{2}}~,\\N&=n+{\frac {\alpha +\beta +1}{2}}~,\\\gamma &=-(\alpha +{\frac {1}{2}}){\frac {\pi }{2}}~,\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/07e6683dc951b008ddd70876151d468f3d6486e4)
Asimptote blizu ±1 dane su sa:
![{\displaystyle {\begin{aligned}\lim _{n\to \infty }n^{-\alpha }P_{n}^{\alpha ,\beta }\left(\cos {\frac {z}{n}}\right)&=\left({\frac {z}{2}}\right)^{-\alpha }J_{\alpha }(z)~,\\\lim _{n\to \infty }n^{-\beta }P_{n}^{\alpha ,\beta }\left(\cos \left[\pi -{\frac {z}{n}}\right]\right)&=\left({\frac {z}{2}}\right)^{-\beta }J_{\beta }(z)~,\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a65b8c5c0053ddaacf22938424f200a6d4a8224e)
Jakobijevi polinomi povezani su sa Vignerovom D-matricom:
![{\displaystyle d_{m'm}^{j}(\phi )=\left[{\frac {(j+m)!(j-m)!}{(j+m')!(j-m')!}}\right]^{1/2}\left(\sin {\frac {\phi }{2}}\right)^{m-m'}\left(\cos {\frac {\phi }{2}}\right)^{m+m'}P_{j-m}^{(m-m',m+m')}(\cos \phi ).}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2abcaf41d9baf29c60160c839c3d9e062c80ce9e)