Laguerre polynomials

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Complex color plot of the Laguerre polynomial L n(x) with n as -1 divided by 9 and x as z to the power of 4 from -2-2i to 2+2i
Complex color plot of the Laguerre polynomial L n(x) with n as -1 divided by 9 and x as z to the power of 4 from -2-2i to 2+2i

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

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

Sometimes the name Laguerre polynomials is used for solutions of

where n is still a non-negative integer. Then they are also named generalized Laguerre polynomials, as will be done here (alternatively associated Laguerre polynomials or, rarely, Sonine polynomials, after their inventor[1] Nikolay Yakovlevich Sonin).

More generally, a Laguerre function is a solution when n is not necessarily a non-negative integer.

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

These polynomials, usually denoted L0L1, …, are a polynomial sequence which may be defined by the Rodrigues formula,

reducing to the closed form of a following section.

They are orthogonal polynomials with respect to an inner product

The sequence of Laguerre polynomials n! Ln is a Sheffer sequence,

The rook polynomials in combinatorics are more or less the same as Laguerre polynomials, up to elementary changes of variables. Further see the Tricomi–Carlitz polynomials.

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 Morse potential and of the 3D isotropic harmonic oscillator.

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

The first few polynomials[edit]

These are the first few Laguerre polynomials:

n
0
1
2
3
4
5
6
n
The first six Laguerre polynomials.

Recursive definition, closed form, and generating function[edit]

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

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

In solution of some boundary value problems, the characteristic values can be useful:

The closed form is

The generating function for them likewise follows,

Polynomials of negative index can be expressed using the ones with positive index:

Relation to binary functions[edit]

There is a method to set Laguerre polynomials using functions which is related to binary expansion of :

Here
with .

Also

Here is A007814 and is a generalisation of A347204.

Generalized Laguerre polynomials[edit]

For arbitrary real α the polynomial solutions of the differential equation[2]

are called generalized Laguerre polynomials, or associated Laguerre polynomials.

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

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

The simple Laguerre polynomials are the special case α = 0 of the generalized Laguerre polynomials:

The Rodrigues formula for them is

The generating function for them is

The first few generalized Laguerre polynomials, Ln(k)(x)

Explicit examples and properties of the generalized Laguerre polynomials[edit]

  • Laguerre functions are defined by confluent hypergeometric functions and Kummer's transformation as[3]
    where is a generalized binomial coefficient. When n is an integer the function reduces to a polynomial of degree n. It has the alternative expression[4]
    in terms of Kummer's function of the second kind.
  • The closed form for these generalized Laguerre polynomials of degree n is[5]
    derived by applying Leibniz's theorem for differentiation of a product to Rodrigues' formula.
  • Laguerre polynomials have a differential operator representation, much like the closely related Hermite polynomials. Namely, let and consider the differential operator . Then .
  • The first few generalized Laguerre polynomials are:
  • The coefficient of the leading term is (−1)n/n!;
  • The constant term, which is the value at 0, is
  • If α is non-negative, then Ln(α) has n real, strictly positive roots (notice that is a Sturm chain), which are all in the interval [citation needed]
  • The polynomials' asymptotic behaviour for large n, but fixed α and x > 0, is given by[6][7]
    and summarizing by
    where is the Bessel function.

As a contour integral[edit]

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

where the contour circles the origin once in a counterclockwise direction without enclosing the essential singularity at 1

Recurrence relations[edit]

The addition formula for Laguerre polynomials:[8]

Laguerre's polynomials satisfy the recurrence relations

in particular
and
or
moreover

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

combined they give this additional, useful recurrence relations

Since is a monic polynomial of degree in , there is the partial fraction decomposition

The second equality follows by the following identity, valid for integer i and n and immediate from the expression of in terms of Charlier polynomials:
For the third equality apply the fourth and fifth identities of this section.

Derivatives of generalized Laguerre polynomials[edit]

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

This points to a special case (α = 0) of the formula above: for integer α = k the generalized polynomial may be written

the shift by k sometimes causing confusion with the usual parenthesis notation for a derivative.

Moreover, the following equation holds:

which generalizes with Cauchy's formula to

The derivative with respect to the second variable α has the form,[9]

This is evident from the contour integral representation below.

The generalized Laguerre polynomials obey the differential equation

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

where for this equation only.

In Sturm–Liouville form the differential equation is

which shows that L(α)
n
is an eigenvector for the eigenvalue n.

Orthogonality[edit]

The generalized Laguerre polynomials are orthogonal over [0, ∞) with respect to the measure with weighting function xα ex:[10]

which follows from

If denotes the gamma distribution then the orthogonality relation can be written as

The associated, symmetric kernel polynomial has the representations (Christoffel–Darboux formula)[citation needed]

recursively

Moreover,[clarification needed Limit as n goes to infinity?]

Turán's inequalities can be derived here, which is

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

Series expansions[edit]

Let a function have the (formal) series expansion

Then

The series converges in the associated Hilbert space L2[0, ∞) if and only if

Further examples of expansions[edit]

Monomials are represented as

while binomials have the parametrization

This leads directly to

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

In quantum mechanics[edit]

In quantum mechanics the Schrödinger equation for the hydrogen-like atom is exactly solvable by separation of variables in spherical coordinates. The radial part of the wave function is a (generalized) Laguerre polynomial.[11]

Vibronic transitions in the Franck-Condon approximation can also be described using Laguerre polynomials.[12]

Multiplication theorems[edit]

Erdélyi gives the following two multiplication theorems [13]

Relation to Hermite polynomials[edit]

The generalized Laguerre polynomials are related to the Hermite polynomials:

where the Hn(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[edit]

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

where is the Pochhammer symbol (which in this case represents the rising factorial).

Hardy–Hille formula[edit]

The generalized Laguerre polynomials satisfy the Hardy–Hille formula[14][15]

where the series on the left converges for and . Using the identity
(see generalized hypergeometric function), this can also be written as
This formula is a generalization of the Mehler kernel for Hermite polynomials, which can be recovered from it by using the relations between Laguerre and Hermite polynomials given above.

Physicist Scaling Convention[edit]

The generalized Laguerre polynomials are used to describe the quantum wavefunction for hydrogen atom orbitals. In the introductory literature on this topic,[16][17][18] a different scaling is used for the generalized Laguerre polynomials than the scaling presented in this article. In the convention taken here, the generalized Laguerre polynomials can be expressed as [19]

where is the confluent hypergeometric function. In the physicist literature, such as [18], the generalized Laguerre polynomials are instead defined as

The physicist version is related to the standard version by

There is yet another convention in use, though less frequently, in the physics literature. Under this convention the Laguerre polynomials are given by [20][21][22]

See also[edit]

Notes[edit]

  1. ^ N. Sonine (1880). "Recherches sur les fonctions cylindriques et le développement des fonctions continues en séries". Math. Ann. 16 (1): 1–80. doi:10.1007/BF01459227. S2CID 121602983.
  2. ^ A&S p. 781
  3. ^ A&S p. 509
  4. ^ A&S p. 510
  5. ^ A&S p. 775
  6. ^ Szegő, p. 198.
  7. ^ D. Borwein, J. M. Borwein, R. E. Crandall, "Effective Laguerre asymptotics", SIAM J. Numer. Anal., vol. 46 (2008), no. 6, pp. 3285–3312 doi:10.1137/07068031X
  8. ^ A&S equation (22.12.6), p. 785
  9. ^ Koepf, Wolfram (1997). "Identities for families of orthogonal polynomials and special functions". Integral Transforms and Special Functions. 5 (1–2): 69–102. CiteSeerX 10.1.1.298.7657. doi:10.1080/10652469708819127.
  10. ^ "Associated Laguerre Polynomial".
  11. ^ Ratner, Schatz, Mark A., George C. (2001). Quantum Mechanics in Chemistry. 0-13-895491-7: Prentice Hall. pp. 90–91.{{cite book}}: CS1 maint: location (link)
  12. ^ Jong, Mathijs de; Seijo, Luis; Meijerink, Andries; Rabouw, Freddy T. (2015-06-24). "Resolving the ambiguity in the relation between Stokes shift and Huang–Rhys parameter". Physical Chemistry Chemical Physics. 17 (26): 16959–16969. Bibcode:2015PCCP...1716959D. doi:10.1039/C5CP02093J. hdl:1874/321453. ISSN 1463-9084. PMID 26062123.
  13. ^ C. Truesdell, "On the Addition and Multiplication Theorems for the Special Functions", Proceedings of the National Academy of Sciences, Mathematics, (1950) pp. 752–757.
  14. ^ Szegő, p. 102.
  15. ^ W. A. Al-Salam (1964), "Operational representations for Laguerre and other polynomials", Duke Math J. 31 (1): 127–142.
  16. ^ Griffiths, David J. (2005). Introduction to quantum mechanics (2nd ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 0131118927.
  17. ^ Sakurai, J. J. (2011). Modern quantum mechanics (2nd ed.). Boston: Addison-Wesley. ISBN 978-0805382914.
  18. ^ a b Merzbacher, Eugen (1998). Quantum mechanics (3rd ed.). New York: Wiley. ISBN 0471887021.
  19. ^ Abramowitz, Milton (1965). Handbook of mathematical functions, with formulas, graphs, and mathematical tables. New York: Dover Publications. ISBN 978-0-486-61272-0.
  20. ^ Schiff, Leonard I. (1968). Quantum mechanics (3d ed.). New York: McGraw-Hill. ISBN 0070856435.
  21. ^ Messiah, Albert (2014). Quantum Mechanics. Dover Publications. ISBN 9780486784557.
  22. ^ Boas, Mary L. (2006). Mathematical methods in the physical sciences (3rd ed.). Hoboken, NJ: Wiley. ISBN 9780471198260.

References[edit]

External links[edit]