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In mathematics, the Laguerre polynomials, named after Edmond Laguerre (1834 – 1886), are solutions of Laguerre's equation:

$x\,y''+(1-x)\,y'+n\,y=0\,$

which is a second-order linear differential equation. This equation has nonsingular solutions only if n is a non-negative integer.

The associated Laguerre polynomials (alternatively, but rarely, named Sonin polynomials, after their inventor N. Y. Sonin) are solutions of

$x\,y''+(\alpha +1-x)\,y'+n\,y=0~.$

The Laguerre polynomials are also used for Gaussian quadrature to numerically compute integrals of the form

$\int _{0}^{\infty }f(x)e^{-x}\,dx.$

These polynomials, usually denoted L₀, L₁, ..., are a polynomial sequence which may be defined by the Rodrigues formula,

$L_{n}(x)={\frac {e^{x}}{n!}}{\frac {d^{n}}{dx^{n}}}\left(e^{-x}x^{n}\right)={\frac {({\frac {d}{dx}}-1)^{n}}{n!}}x^{n},$

reducing to the closed form of a following section.

They are orthogonal polynomials with respect to an inner product

$\langle f,g\rangle =\int _{0}^{\infty }f(x)g(x)e^{-x}\,dx.$

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The Rook polynomials in combinatorics are more or less the same as Laguerre polynomials, up to elementary changes of variables.

The Laguerre polynomials arise in quantum mechanics, in the radial part of the solution of the Schrödinger equation for a one-electron atom. They also describe the static Wigner functions of oscillator systems in quantum mechanics in phase space. They further enter in the quantum mechanics of the 3D isotropic harmonic oscillator.

Physicists sometimes use a definition for the Laguerre polynomials which is larger by a factor of n! than the definition used here. (Likewise, some physicist may use somewhat different definitions of the so-called associated Laguerre polynomials.)

The first few polynomials

These are the first few Laguerre polynomials:

n	$L_{n}(x)\,$
0	$1\,$
1	$-x+1\,$
2	${\scriptstyle {\frac {1}{2}}}(x^{2}-4x+2)\,$
3	${\scriptstyle {\frac {1}{6}}}(-x^{3}+9x^{2}-18x+6)\,$
4	${\scriptstyle {\frac {1}{24}}}(x^{4}-16x^{3}+72x^{2}-96x+24)\,$
5	${\scriptstyle {\frac {1}{120}}}(-x^{5}+25x^{4}-200x^{3}+600x^{2}-600x+120)\,$
6	${\scriptstyle {\frac {1}{720}}}(x^{6}-36x^{5}+450x^{4}-2400x^{3}+5400x^{2}-4320x+720)\,$

The first six Laguerre polynomials.

Recursive definition, closed form, and generating function

One can also define the Laguerre polynomials recursively, defining the first two polynomials as

$L_{0}(x)=1\,$

$L_{1}(x)=1-x\,$

and then using the following recurrence relation for any k ≥ 1:

$L_{k+1}(x)={\frac {1}{k+1}}\left((2k+1-x)L_{k}(x)-kL_{k-1}(x)\right).$

The closed form is

$L_{n}(x)=\sum _{k=0}^{n}{\binom {n}{k}}{\frac {(-1)^{k}}{k!}}x^{k}.$

The generating function for them likewise follows,

$\sum _{n}^{\infty }t^{n}L_{n}(x)={\frac {1}{1-t}}~e^{\frac {-tx}{1-t}}~.$

Generalized Laguerre polynomials

For arbitrary real α the polynomial solutions of the differential equation

$x\,y''+(\alpha +1-x)\,y'+n\,y=0$

are called generalized Laguerre polynomials, or associated Laguerre polynomials.

The simple Laguerre polynomials are included in the associated polynomials, through α = 0,

$L_{n}^{(0)}(x)=L_{n}(x).$

The Rodrigues formula for them is

$L_{n}^{(\alpha )}(x)={x^{-\alpha }e^{x} \over n!}{d^{n} \over dx^{n}}\left(e^{-x}x^{n+\alpha }\right)=x^{-\alpha }~{\frac {({\frac {d}{dx}}-1)^{n}}{n!}}~x^{n+\alpha }.$

The generating function for them is

$\sum _{n}^{\infty }t^{n}L_{n}^{(\alpha )}(x)={\frac {1}{(1-t)^{\alpha +1}}}~e^{\frac {-tx}{1-t}}~.$

Explicit examples and properties of the associated Laguerre polynomials

• Laguerre functions are defined by confluent hypergeometric functions and Kummer's transformation as

$L_{n}^{(\alpha )}(x):={n+\alpha \choose n}M(-n,\alpha +1,x).$

When n is an integer the function reduces to a polynomial of degree n. It has the alternative expression

$L_{n}^{(\alpha )}(x)={\frac {(-1)^{n}}{n!}}U(-n,\alpha +1,x)$

in terms of Kummer's function of the second kind.

• The closed form for these associated Laguerre polynomials of degree n is

$L_{n}^{(\alpha )}(x)=\sum _{i=0}^{n}(-1)^{i}{n+\alpha \choose n-i}{\frac {x^{i}}{i!}}$

derived by applying Leibniz's theorem for differentiation of a product to Rodrigues' formula.

• The first few generalized Laguerre polynomials are:

${\begin{aligned}L_{0}^{(\alpha )}(x)&=1\\L_{1}^{(\alpha )}(x)&=-x+\alpha +1\\L_{2}^{(\alpha )}(x)&={\frac {x^{2}}{2}}-(\alpha +2)x+{\frac {(\alpha +2)(\alpha +1)}{2}}\\L_{3}^{(\alpha )}(x)&={\frac {-x^{3}}{6}}+{\frac {(\alpha +3)x^{2}}{2}}-{\frac {(\alpha +2)(\alpha +3)x}{2}}+{\frac {(\alpha +1)(\alpha +2)(\alpha +3)}{6}}\end{aligned}}$

• The coefficient of the leading term is (−1)ⁿ/n!;
• The constant term, which is the value at 0, is

$L_{n}^{(\alpha )}(0)={n+\alpha \choose n}\approx {\frac {n^{\alpha }}{\Gamma (\alpha +1)}};$

• L_{n^(α) has n real, strictly positive roots (notice that $\left((-1)^{n-i}L_{n-i}^{(\alpha )}\right)_{i=0}^{n}$ is a Sturm chain), which are all in the interval $\left(0,n+\alpha +(n-1){\sqrt {n+\alpha }}\right].$Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park.}

• The polynomials' asymptotic behaviour for large n, but fixed α and x > 0, is given by

$L_{n}^{(\alpha )}(x)={\frac {n^{{\frac {\alpha }{2}}-{\frac {1}{4}}}}{\sqrt {\pi }}}{\frac {e^{\frac {x}{2}}}{x^{{\frac {\alpha }{2}}+{\frac {1}{4}}}}}\cos \left(2{\sqrt {nx}}-{\frac {\pi }{2}}\left(\alpha +{\frac {1}{2}}\right)\right)+O\left(n^{{\frac {\alpha }{2}}-{\frac {3}{4}}}\right),$

$L_{n}^{(\alpha )}(-x)={\frac {(n+1)^{{\frac {\alpha }{2}}-{\frac {1}{4}}}}{2{\sqrt {\pi }}}}{\frac {e^{-{\frac {x}{2}}}}{x^{{\frac {\alpha }{2}}+{\frac {1}{4}}}}}e^{2{\sqrt {x(n+1)}}}\cdot \left(1+O\left({\frac {1}{\sqrt {n+1}}}\right)\right),$

and summarizing by

${\frac {L_{n}^{(\alpha )}\left({\frac {x}{n}}\right)}{n^{\alpha }}}\approx e^{\frac {x}{2n}}\cdot {\frac {J_{\alpha }\left(2{\sqrt {x}}\right)}{{\sqrt {x}}^{\alpha }}},$

where $J_{\alpha }$ is the Bessel function.

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$L_{n}^{(\alpha -n)}(x)\approx e^{x}\cdot {\alpha \choose n}$,

whenever n tends to infinity.

As a contour integral

Given the generating function specified above, the polynomials may be expressed in terms of a contour integral

$L_{n}^{(\alpha )}(x)={\frac {1}{2\pi i}}\oint {\frac {e^{-{\frac {xt}{1-t}}}}{(1-t)^{\alpha +1}\,t^{n+1}}}\;dt~,$

where the contour circles the origin once in a counterclockwise direction.

Recurrence relations

The addition formula for Laguerre polynomials:

$L_{n}^{(\alpha +\beta +1)}(x+y)=\sum _{i=0}^{n}L_{i}^{(\alpha )}(x)L_{n-i}^{(\beta )}(y)$.

Laguerre's polynomials satisfy the recurrence relations

$L_{n}^{(\alpha )}(x)=\sum _{i=0}^{n}L_{n-i}^{(\alpha +i)}(y){\frac {(y-x)^{i}}{i!}},$

in particular

$L_{n}^{(\alpha +1)}(x)=\sum _{i=0}^{n}L_{i}^{(\alpha )}(x)$

and

$L_{n}^{(\alpha )}(x)=\sum _{i=0}^{n}{\alpha -\beta +n-i-1 \choose n-i}L_{i}^{(\beta )}(x),$

or

$L_{n}^{(\alpha )}(x)=\sum _{i=0}^{n}{\alpha -\beta +n \choose n-i}L_{i}^{(\beta -i)}(x);$

moreover

${\begin{aligned}L_{n}^{(\alpha )}(x)-\sum _{j=0}^{\Delta -1}{n+\alpha \choose n-j}(-1)^{j}{\frac {x^{j}}{j!}}&=(-1)^{\Delta }{\frac {x^{\Delta }}{(\Delta -1)!}}\sum _{i=0}^{n-\Delta }{\frac {n+\alpha \choose n-\Delta -i}{(n-i){n \choose i}}}L_{i}^{(\alpha +\Delta )}(x)\\[6pt]&=(-1)^{\Delta }{\frac {x^{\Delta }}{(\Delta -1)!}}\sum _{i=0}^{n-\Delta }{\frac {n+\alpha -i-1 \choose n-\Delta -i}{(n-i){n \choose i}}}L_{i}^{(n+\alpha +\Delta -i)}(x).\end{aligned}}$

They can be used to derive the four 3-point-rules

${\begin{aligned}L_{n}^{(\alpha )}(x)&=L_{n}^{(\alpha +1)}(x)-L_{n-1}^{(\alpha +1)}(x)=\sum _{j=0}^{k}{k \choose j}L_{n-j}^{(\alpha -k+j)}(x),\\[10pt]nL_{n}^{(\alpha )}(x)&=(n+\alpha )L_{n-1}^{(\alpha )}(x)-xL_{n-1}^{(\alpha +1)}(x),\\[10pt]&{\text{or }}{\frac {x^{k}}{k!}}L_{n}^{(\alpha )}(x)=\sum _{i=0}^{k}(-1)^{i}{n+i \choose i}{n+\alpha \choose k-i}L_{n+i}^{(\alpha -k)}(x),\\[10pt]nL_{n}^{(\alpha +1)}(x)&=(n-x)L_{n-1}^{(\alpha +1)}(x)+(n+\alpha )L_{n-1}^{(\alpha )}(x)\\[10pt]xL_{n}^{(\alpha +1)}(x)&=(n+\alpha )L_{n-1}^{(\alpha )}(x)-(n-x)L_{n}^{(\alpha )}(x);\end{aligned}}$

combined they give this additional, useful recurrence relations

${\begin{aligned}L_{n}^{(\alpha )}(x)&=\left(2+{\frac {\alpha -1-x}{n}}\right)L_{n-1}^{(\alpha )}(x)-\left(1+{\frac {\alpha -1}{n}}\right)L_{n-2}^{(\alpha )}(x)\\[10pt]&={\frac {\alpha +1-x}{n}}L_{n-1}^{(\alpha +1)}(x)-{\frac {x}{n}}L_{n-2}^{(\alpha +2)}(x).\end{aligned}}$

A somewhat curious identity, valid for integer i and n, is

${\frac {(-x)^{i}}{i!}}L_{n}^{(i-n)}(x)={\frac {(-x)^{n}}{n!}}L_{i}^{(n-i)}(x);$

it may be used to derive the partial fraction decomposition

${\frac {L_{n}^{(\alpha )}(x)}{n+\alpha \choose n}}=1-\sum _{j=1}^{n}{\frac {x^{j}}{\alpha +j}}{\frac {L_{n-j}^{(j)}(x)}{(j-1)!}}=1-\sum _{j=1}^{n}(-1)^{j}{\frac {j}{\alpha +j}}{n \choose j}L_{n}^{(-j)}(x)=1-x\sum _{i=1}^{n}{\frac {L_{n-i}^{(-\alpha )}(x)L_{i-1}^{(\alpha +1)}(-x)}{\alpha +i}}.$

Derivatives of generalized Laguerre polynomials

Differentiating the power series representation of a generalized Laguerre polynomial k times leads to

${\frac {d^{k}}{dx^{k}}}L_{n}^{(\alpha )}(x)=(-1)^{k}L_{n-k}^{(\alpha +k)}(x)\,.$

This points to a special case (α = 0) of the formula above: for integer α = k the generalized polynomial may be written $L_{n}^{(k)}(x)=(-1)^{k}{\frac {d^{k}L_{n+k}(x)}{dx^{k}}}\,$, the shift by k sometimes causing confusion with the usual parenthesis notation for a derivative.

Moreover, this following equation holds

${\frac {1}{k!}}{\frac {d^{k}}{dx^{k}}}x^{\alpha }L_{n}^{(\alpha )}(x)={n+\alpha \choose k}x^{\alpha -k}L_{n}^{(\alpha -k)}(x),$

which generalizes with Cauchy's formula to

$L_{n}^{(\alpha ')}(x)=(\alpha '-\alpha ){\alpha '+n \choose \alpha '-\alpha }\int _{0}^{x}{\frac {t^{\alpha }(x-t)^{\alpha '-\alpha -1}}{x^{\alpha '}}}L_{n}^{(\alpha )}(t)\,dt.$

The derivative with respect to the second variable α has the form,

${\frac {d}{d\alpha }}L_{n}^{(\alpha )}(x)=\sum _{i=0}^{n-1}{\frac {L_{i}^{(\alpha )}(x)}{n-i}}.$

This is evident from the contour integral representation below.

The generalized associated Laguerre polynomials obey the differential equation

$xL_{n}^{(\alpha )\prime \prime }(x)+(\alpha +1-x)L_{n}^{(\alpha )\prime }(x)+nL_{n}^{(\alpha )}(x)=0,\,$

which may be compared with the equation obeyed by the kth derivative of the ordinary Laguerre polynomial,

$xL_{n}^{(k)\prime \prime }(x)+(k+1-x)L_{n}^{(k)\prime }(x)+(n-k)L_{n}^{(k)}(x)=0,\,$

where $L_{n}^{(k)}(x)\equiv {\frac {d^{k}L_{n}(x)}{dx^{k}}}$ for this equation only.

In Sturm–Liouville form the differential equation is

$-\left(x^{\alpha +1}e^{-x}\cdot L_{n}^{(\alpha )}(x)^{\prime }\right)^{\prime }=n\cdot x^{\alpha }e^{-x}\cdot L_{n}^{(\alpha )}(x),$

which shows that LTemplate:Su is an eigenvector for the eigenvalue n.

Orthogonality

The associated Laguerre polynomials are orthogonal over [0, ∞) with respect to the measure with weighting function x^α e ^−x:

$\int _{0}^{\infty }x^{\alpha }e^{-x}L_{n}^{(\alpha )}(x)L_{m}^{(\alpha )}(x)dx={\frac {\Gamma (n+\alpha +1)}{n!}}\delta _{n,m},$

which follows from

$\int _{0}^{\infty }x^{\alpha '-1}e^{-x}L_{n}^{(\alpha )}(x)dx={\alpha -\alpha '+n \choose n}\Gamma (\alpha ').$

If $\Gamma (x,\alpha +1,1)$ denoted the Gamma distribution then the orthogonality relation can be written as

$\int _{0}^{\infty }L_{n}^{(\alpha )}(x)L_{m}^{(\alpha )}(x)\Gamma (x,\alpha +1,1)dx={n+\alpha \choose n}\delta _{n,m},$

The associated, symmetric kernel polynomial has the representations (Christoffel–Darboux formula)Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park.

${\begin{aligned}K_{n}^{(\alpha )}(x,y)&{:=}{\frac {1}{\Gamma (\alpha +1)}}\sum _{i=0}^{n}{\frac {L_{i}^{(\alpha )}(x)L_{i}^{(\alpha )}(y)}{\alpha +i \choose i}}\\&{=}{\frac {1}{\Gamma (\alpha +1)}}{\frac {L_{n}^{(\alpha )}(x)L_{n+1}^{(\alpha )}(y)-L_{n+1}^{(\alpha )}(x)L_{n}^{(\alpha )}(y)}{{\frac {x-y}{n+1}}{n+\alpha \choose n}}}\\&{=}{\frac {1}{\Gamma (\alpha +1)}}\sum _{i=0}^{n}{\frac {x^{i}}{i!}}{\frac {L_{n-i}^{(\alpha +i)}(x)L_{n-i}^{(\alpha +i+1)}(y)}{{\alpha +n \choose n}{n \choose i}}};\end{aligned}}$

recursively

$K_{n}^{(\alpha )}(x,y)={\frac {y}{\alpha +1}}K_{n-1}^{(\alpha +1)}(x,y)+{\frac {1}{\Gamma (\alpha +1)}}{\frac {L_{n}^{(\alpha +1)}(x)L_{n}^{(\alpha )}(y)}{\alpha +n \choose n}}.$

Moreover,

$y^{\alpha }e^{-y}K_{n}^{(\alpha )}(\cdot ,y)\rightarrow \delta (y-\,\cdot ),$

in the associated L²[0, ∞)-space.

Turán's inequalities can be derived here, which is

$L_{n}^{(\alpha )}(x)^{2}-L_{n-1}^{(\alpha )}(x)L_{n+1}^{(\alpha )}(x)=\sum _{k=0}^{n-1}{\frac {\alpha +n-1 \choose n-k}{n{n \choose k}}}L_{k}^{(\alpha -1)}(x)^{2}>0.$

The following integral is needed in the quantum mechanical treatment of the hydrogen atom,

$\int _{0}^{\infty }x^{\alpha +1}e^{-x}\left[L_{n}^{(\alpha )}(x)\right]^{2}dx={\frac {(n+\alpha )!}{n!}}(2n+\alpha +1).$

Series expansions

Let a function have the (formal) series expansion

$f(x)=\sum _{i=0}^{\infty }f_{i}^{(\alpha )}L_{i}^{(\alpha )}(x).$

Then

$f_{i}^{(\alpha )}=\int _{0}^{\infty }{\frac {L_{i}^{(\alpha )}(x)}{i+\alpha \choose i}}\cdot {\frac {x^{\alpha }e^{-x}}{\Gamma (\alpha +1)}}\cdot f(x)\,dx.$

The series converges in the associated Hilbert space $L^{2}[0,\infty )$, iff

$\|f\|_{L^{2}}^{2}:=\int _{0}^{\infty }{\frac {x^{\alpha }e^{-x}}{\Gamma (\alpha +1)}}|f(x)|^{2}dx=\sum _{i=0}^{\infty }{i+\alpha \choose i}|f_{i}^{(\alpha )}|^{2}<\infty .$

Further examples of expansions

Monomials are represented as

${\frac {x^{n}}{n!}}=\sum _{i=0}^{n}(-1)^{i}{n+\alpha \choose n-i}L_{i}^{(\alpha )}(x),$

while binomials have the parametrization

${n+x \choose n}=\sum _{i=0}^{n}{\frac {\alpha ^{i}}{i!}}L_{n-i}^{(x+i)}(\alpha ).$

This leads directly to

$e^{-\gamma x}=\sum _{i=0}^{\infty }{\frac {\gamma ^{i}}{(1+\gamma )^{i+\alpha +1}}}L_{i}^{(\alpha )}(x)\qquad \left({\text{convergent iff }}\operatorname {Re} {(\gamma )}>-{\frac {1}{2}}\right)$

for the exponential function. The incomplete gamma function has the representation

$\Gamma (\alpha ,x)=x^{\alpha }e^{-x}\sum _{i=0}^{\infty }{\frac {L_{i}^{(\alpha )}(x)}{1+i}}\qquad \left(\Re (\alpha )>-1,x>0\right).$

Multiplication theorems

Erdélyi gives the following two multiplication theorems

• $t^{n+1+\alpha }e^{(1-t)z}L_{n}^{(\alpha )}(zt)=\sum _{k=n}{k \choose n}\left(1-{\frac {1}{t}}\right)^{k-n}L_{k}^{(\alpha )}(z),$
• $e^{(1-t)z}L_{n}^{(\alpha )}(zt)=\sum _{k=0}{\frac {(1-t)^{k}z^{k}}{k!}}L_{n}^{(\alpha +k)}(z).$

Relation to Hermite polynomials

The generalized Laguerre polynomials are related to the Hermite polynomials:

$H_{2n}(x)=(-1)^{n}\ 2^{2n}\ n!\ L_{n}^{(-1/2)}(x^{2})$

and

$H_{2n+1}(x)=(-1)^{n}\ 2^{2n+1}\ n!\ x\ L_{n}^{(1/2)}(x^{2})$

where the H_n(x) are the Hermite polynomials based on the weighting function exp(−x²), the so-called "physicist's version."

Because of this, the generalized Laguerre polynomials arise in the treatment of the quantum harmonic oscillator.

Relation to hypergeometric functions

The Laguerre polynomials may be defined in terms of hypergeometric functions, specifically the confluent hypergeometric functions, as

$L_{n}^{(\alpha )}(x)={n+\alpha \choose n}M(-n,\alpha +1,x)={\frac {(\alpha +1)_{n}}{n!}}\,_{1}F_{1}(-n,\alpha +1,x)$

where $(a)_{n}$ is the Pochhammer symbol (which in this case represents the rising factorial).

Poisson Kernel

$\sum _{n=0}^{\infty }{\frac {n!L_{n}^{(\alpha )}(x)L_{n}^{(\alpha )}(y)r^{n}}{\Gamma \left(1+\alpha +n\right)}}={\frac {\exp \left(-{\frac {\left(x+y\right)r}{1-r}}\right)I_{\alpha }\left({\frac {2{\sqrt {xyr}}}{1-r}}\right)}{\left(xyr\right)^{\frac {\alpha }{2}}\left(1-r\right)}},\quad ,\alpha >-1,\left|r\right|<1.$

References

• Template:Abramowitz Stegun ref

• Template:Dlmf

• B. Spain, M.G. Smith, Functions of mathematical physics, Van Nostrand Reinhold Company, London, 1970. Chapter 10 deals with Laguerre polynomials.
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• Eric W. Weisstein, "Laguerre Polynomial", From MathWorld—A Wolfram Web Resource.
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• S. S. Bayin (2006), Mathematical Methods in Science and Engineering, Wiley, Chapter 3.

External links

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  Hostgator tutorials on how to install Wordpress need not be complicated, especially when you will be dealing with a web hosting service that is friendly for novice webmasters and a blogging platform that is as intuitive as riding a bike. After that you can get Hostgator to host your domain and use the wordpress to do the blogging. Once you start site flipping, trust me you will not be able to stop. I cut my webmaster teeth on Control Panel many years ago, but since had left for other hosting companies with more commercial (cough, cough) interfaces. If you don't like it, you can chalk it up to experience and go on. First, find a good starter template design. When I signed up, I did a search for current "HostGator codes" on the web, which enabled me to receive a one-word entry for a discount. Your posts, comments, and pictures will all be imported into your new WordPress blog.