# Rodrigues' formula

In mathematics, Rodrigues' formula (formerly called the Ivory–Jacobi formula) is a formula for the Legendre polynomials independently introduced by Olinde Rodrigues (1816), Sir James Ivory (1824) and Carl Gustav Jacobi (1827). The name "Rodrigues formula" was introduced by Heine in 1878, after Hermite pointed out in 1865 that Rodrigues was the first to discover it. The term is also used to describe similar formulas for other orthogonal polynomials. Askey (2005) describes the history of the Rodrigues formula in detail.

## Statement

Let ${\displaystyle \{P_{n}(x)\}_{n=0}^{\infty }}$ be a sequence of orthogonal polynomials satisfying the orthogonality condition

${\displaystyle \int _{a}^{b}P_{m}(x)P_{n}(x)w(x)dx=K_{m,n}\delta _{m,n},}$

where, ${\displaystyle w(x)}$ is a suitable weight function, ${\displaystyle K_{m,n}}$ are constants and ${\displaystyle \delta _{m,n}}$ is the Kronecker delta. If the weight function ${\displaystyle w(x)}$ satisfies the following differential equation (called Pearson's differential equation),

${\displaystyle {\frac {w'(x)}{w(x)}}={\frac {A(x)}{B(x)}},}$

where ${\displaystyle A(x)}$ is a polynomial with degree at most 1 and ${\displaystyle B(x)}$ is a polynomial with degree at most 2 and, further, the limits

${\displaystyle \lim _{x\rightarrow a}w(x)B(x)=0,\qquad \lim _{x\rightarrow b}w(x)B(x)=0,}$

then, it can be shown that ${\displaystyle P_{n}(x)}$ satisfies a recurrence relation of the form,

${\displaystyle P_{n}(x)={\frac {c_{n}}{w(x)}}{\frac {\mathrm {d} ^{n}}{\mathrm {d} x^{n}}}\left[B(x)^{n}w(x)\right],}$

for a given constants ${\displaystyle c_{n}}$. This relation is called Rodrigues' type formula, or just Rodrigues' formula.[1]

The most known applications of Rodrigues' type formulas are the formulas for Legendre, Laguerre and Hermite polynomials:

Rodrigues stated his formula for Legendre polynomials ${\displaystyle P_{n}}$:

${\displaystyle P_{n}(x)={1 \over 2^{n}n!}{d^{n} \over dx^{n}}\left[(x^{2}-1)^{n}\right].}$

Laguerre polynomials are usually denoted L0L1, ..., and the Rodrigues formula can be written as

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

The Rodrigues formula for the Hermite polynomial can be written as

${\displaystyle H_{n}(x)=(-1)^{n}e^{x^{2}}{\frac {d^{n}}{dx^{n}}}e^{-x^{2}}=\left(2x-{\frac {d}{dx}}\right)^{n}\cdot 1}$.

Similar formulae hold for many other sequences of orthogonal functions arising from Sturm-Liouville equations, and these are also called the Rodrigues formula (or Rodrigues' type formula) for that case, especially when the resulting sequence is polynomial.

## References

1. ^ "Rodrigues formula - Encyclopedia of Mathematics". www.encyclopediaofmath.org. Retrieved 2018-04-18.