18 Orthogonal PolynomialsApplications18.37 Classical OP’s in Two or More Variables18.39 Physical Applications

- §18.38(i) Classical OP’s: Numerical Analysis
- §18.38(ii) Classical OP’s: Other Applications
- §18.38(iii) Other OP’s

The scaled Chebyshev polynomial ${2}^{1-n}{T}_{n}\left(x\right)$, $n\ge 1$, enjoys the “minimax” property on the interval $\left[-1,1\right]$, that is, $\left|{2}^{1-n}{T}_{n}\left(x\right)\right|$ has the least maximum value among all monic polynomials of degree $n$. In consequence, expansions of functions that are infinitely differentiable on $\left[-1,1\right]$ in series of Chebyshev polynomials usually converge extremely rapidly. For these results and applications in approximation theory see §3.11(ii) and Mason and Handscomb (2003, Chapter 3), Cheney (1982, p. 108), and Rivlin (1969, p. 31).

Classical OP’s play a fundamental role in Gaussian quadrature. If the nodes in a quadrature formula with a positive weight function are chosen to be the zeros of the $n$th degree OP with the same weight function, and the interval of orthogonality is the same as the integration range, then the weights in the quadrature formula can be chosen in such a way that the formula is exact for all polynomials of degree not exceeding $2n-1$. See §3.5(v).

Linear ordinary differential equations can be solved directly in series of Chebyshev polynomials (or other OP’s) by a method originated by Clenshaw (1957). This process has been generalized to spectral methods for solving partial differential equations. For further information see Mason and Handscomb (2003, Chapters 10 and 11), Gottlieb and Orszag (1977, pp. 7–19), and Guo (1998, pp. 120–151).

The Toda equation provides an important model of a completely integrable system. It has elegant structures, including $N$-soliton solutions, Lax pairs, and Bäcklund transformations. While the Toda equation is an important model of nonlinear systems, the special functions of mathematical physics are usually regarded as solutions to linear equations. However, by using Hirota’s technique of bilinear formalism of soliton theory, Nakamura (1996) shows that a wide class of exact solutions of the Toda equation can be expressed in terms of various special functions, and in particular classical OP’s. For instance,

18.38.1 | $${V}_{n}\left(x\right)=2n{H}_{n+1}\left(x\right){H}_{n-1}\left(x\right)/{\left({H}_{n}\left(x\right)\right)}^{2},$$ | ||

with ${H}_{n}\left(x\right)$ as in §18.3, satisfies the Toda equation

18.38.2 | $$\left({\mathrm{d}}^{2}/{\mathrm{d}x}^{2}\right)\mathrm{ln}{V}_{n}\left(x\right)={V}_{n+1}\left(x\right)+{V}_{n-1}\left(x\right)-2{V}_{n}\left(x\right),$$ | ||

$n=1,2,\mathrm{\dots}$. | |||

The Askey–Gasper inequality

18.38.3 | $$\sum _{m=0}^{n}{P}_{m}^{\left(\alpha ,0\right)}\left(x\right)\ge 0,$$ | ||

$-1\le x\le 1$, $\alpha >-1$, $n=0,1,\mathrm{\dots}$, | |||

was used in de Branges’ proof of the long-standing Bieberbach conjecture concerning univalent functions on the unit disk in the complex plane. See de Branges (1985).

See Deans (1983, Chapters 4, 7).