Mehler–Heine formula
{{Short description|Formula describing the asymptotic behavior of the Legendre polynomials}}
In mathematics, the Mehler–Heine formula introduced by Gustav Ferdinand Mehler{{Cite journal |last=Mehler |first=G.F. |date=1868 |title=Ueber die Vertheilung der statischen Elektricität in einem von zwei Kugelkalotten begrenzten Körper |url=https://zenodo.org/record/1448892/files/article.pdf |journal=Journal für die Reine und Angewandte Mathematik |volume=68 |pages=134-150}} and Eduard Heine{{Cite book |last=Heine |first=E. |url=https://books.google.com/books?id=D79hEMl2GM0C |title=Handbuch der Kugelfunktionen. Theorie und Anwendung. |publisher=Georg Reimer |year=1861 |location=Berlin}} describes the asymptotic behavior of the Legendre polynomials as the index tends to infinity, near the edges of the support of the weight. There are generalizations to other classical orthogonal polynomials, which are also called the Mehler–Heine formula. The formula complements the Darboux formulae which describe the asymptotics in the interior and outside the support.
Legendre polynomials
The simplest case of the Mehler–Heine formula states that
:
= \lim _{n\to\infty}P_n\left(1-\frac{z^2}{2n^2}\right)
= J_0(z),
where {{math|Pn}} is the Legendre polynomial of order {{mvar|n}}, and {{math|J0}} the Bessel function of order 0. The limit is uniform over {{mvar|z}} in an arbitrary bounded domain in the complex plane.
Jacobi polynomials
The generalization to Jacobi polynomials {{math|P{{su|b=n|p=(α, β)}}}} is given by Gábor Szegő{{Cite book |last=Szegő |first=Gábor |title=Orthogonal Polynomials |publisher=American Mathematical Society |year=1939 |isbn=978-0-8218-1023-1 |series=Colloquium Publications |mr=0372517}} as follows
:
= \lim_{n \to \infty} n^{-\alpha}P_n^{(\alpha,\beta)}\left(1-\frac{z^2}{2n^2}\right)
= \left(\frac{z}{2}\right)^{-\alpha} J_\alpha(z),
where {{math|Jα}} is the Bessel function of Bessel_function#Bessel_functions_of_the_first_kind:_Jα.
Laguerre polynomials
Using generalized Laguerre polynomials and confluent hypergeometric functions, they can be written as
:
= \left(\frac{z}{2}\right)^{-\alpha} J_\alpha(z),
where {{math|L{{su|b=n|p=(α)}}}} is the Laguerre function.
Hermite polynomials
Using the expressions equivalating Hermite polynomials and Laguerre polynomials where two equations exist,{{Cite book |last=Koekoek |first=Roelof |title=Hypergeometric Orthogonal Polynomials and Their q-Analogues |last2=Lesky |first2=P.A. |last3=Swarttouw |first3=R.F. |publisher=Springer-Verlag |year=2010 |isbn=978-3-642-05013-8 |doi=10.1007/978-3-642-05014-5}} they can be written as
:
&=\left(\frac{z}{2}\right)^{\frac{1}{2}}J_{-\frac{1}{2}}(z) \\
\lim_{n \to \infty} \frac{(-1)^n}{4^nn!}H_{2n+1}\left(\frac{z}{2\sqrt{n}}\right)
&=(2z)^{\frac{1}{2}}J_{\frac{1}{2}}(z),\end{align}
where {{math|Hn}} is the Hermite function.