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In physical science and mathematics, the Legendre functions , and associated Legendre functions , , and Legendre functions of the second kind, , are all solutions of Legendre's differential equation. The
Legendre polynomials In physical science and mathematics, Legendre polynomials (named after Adrien-Marie Legendre, who discovered them in 1782) are a system of complete and orthogonal polynomials, with a vast number of mathematical properties, and numerous applicat ...
and the associated Legendre polynomials are also solutions of the differential equation in special cases, which, by virtue of being polynomials, have a large number of additional properties, mathematical structure, and applications. For these polynomial solutions, see the separate Wikipedia articles.


Legendre's differential equation

The general Legendre equation reads \left(1 - x^2\right) y'' - 2xy' + \left lambda(\lambda+1) - \frac\righty = 0, where the numbers and may be complex, and are called the degree and order of the relevant function, respectively. The polynomial solutions when is an integer (denoted ), and are the Legendre polynomials ; and when is an integer (denoted ), and is also an integer with are the associated Legendre polynomials. All other cases of and can be discussed as one, and the solutions are written , . If , the superscript is omitted, and one writes just , . However, the solution when is an integer is often discussed separately as Legendre's function of the second kind, and denoted . This is a second order linear equation with three regular singular points (at , , and ). Like all such equations, it can be converted into a hypergeometric differential equation by a change of variable, and its solutions can be expressed using hypergeometric functions.


Solutions of the differential equation

Since the differential equation is linear, homogeneous (the right hand side =zero) and of second order, it has two linearly independent solutions, which can both be expressed in terms of the hypergeometric function, _2F_1. With \Gamma being the
gamma function In mathematics, the gamma function (represented by , the capital letter gamma from the Greek alphabet) is one commonly used extension of the factorial function to complex numbers. The gamma function is defined for all complex numbers except ...
, the first solution is P_^(z) = \frac \left frac\right \,_2F_1 \left(-\lambda, \lambda+1; 1-\mu; \frac\right),\qquad \text \ , 1-z, <2 and the second is, Q_^(z) = \frac\frac \,_2F_1 \left(\frac, \frac; \lambda+\frac; \frac\right),\qquad \text\ \ , z, >1. These are generally known as Legendre functions of the first and second kind of noninteger degree, with the additional qualifier 'associated' if is non-zero. A useful relation between the and solutions is Whipple's formula.


Legendre functions of the second kind ()

The nonpolynomial solution for the special case of integer degree \lambda = n \in \N_0 , and \mu = 0 , is often discussed separately. It is given by Q_n(x)=\frac\left(x^+\fracx^+\fracx^+\cdots\right) This solution is necessarily
singular Singular may refer to: * Singular, the grammatical number that denotes a unit quantity, as opposed to the plural and other forms * Singular homology * SINGULAR, an open source Computer Algebra System (CAS) * Singular or sounder, a group of boar, ...
when x = \pm 1 . The Legendre functions of the second kind can also be defined recursively via Bonnet's recursion formula Q_n(x) = \begin \frac \log \frac & n = 0 \\ P_1(x) Q_0(x) - 1 & n = 1 \\ \frac x Q_(x) - \frac Q_(x) & n \geq 2 \,. \end


Associated Legendre functions of the second kind

The nonpolynomial solution for the special case of integer degree \lambda = n \in \N_0 , and \mu = m \in \N_0 is given by Q_n^(x) = (-1)^m (1-x^2)^\frac \fracQ_n(x)\,.


Integral representations

The Legendre functions can be written as contour integrals. For example, P_\lambda(z) =P^0_\lambda(z) = \frac \int_ \fracdt where the contour winds around the points and in the positive direction and does not wind around . For real , we have P_s(x) = \frac\int_^\left(x+\sqrt\cos\theta\right)^s d\theta = \frac\int_0^1\left(x+\sqrt(2t-1)\right)^s\frac,\qquad s\in\Complex


Legendre function as characters

The real integral representation of P_s are very useful in the study of harmonic analysis on L^1(G//K) where G//K is the
double coset space A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * ...
of SL(2,\R) (see
Zonal spherical function In mathematics, a zonal spherical function or often just spherical function is a function on a locally compact group ''G'' with compact subgroup ''K'' (often a maximal compact subgroup) that arises as the matrix coefficient of a ''K''-invariant vect ...
). Actually the Fourier transform on L^1(G//K) is given by L^1(G//K)\ni f\mapsto \hat where \hat(s)=\int_1^\infty f(x)P_s(x)dx,\qquad -1\leq\Re(s)\leq 0


See also

* Ferrers function


References

* * . * * * *


External links


Legendre function P
on the Wolfram functions site.
Legendre function Q
on the Wolfram functions site.
Associated Legendre function P
on the Wolfram functions site.
Associated Legendre function Q
on the Wolfram functions site. {{Authority control Hypergeometric functions