# Lagrange reversion theorem

In mathematics, the Lagrange reversion theorem gives series or formal power series expansions of certain implicitly defined functions; indeed, of compositions with such functions.

Let v be a function of x and y in terms of another function f such that

$v=x+yf(v)$

Then for any function g,

$g(v)=g(x)+\sum_{k=1}^\infty\frac{y^k}{k!}\left(\frac\partial{\partial x}\right)^{k-1}\left(f(x)^kg'(x)\right)$

for small y. If g is the identity

$v=x+\sum_{k=1}^\infty\frac{y^k}{k!}\left(\frac\partial{\partial x}\right)^{k-1}\left(f(x)^k\right)$

In 1770, Joseph Louis Lagrange (1736–1813) published his power series solution of the implicit equation for v mentioned above. However, his solution used cumbersome series expansions of logarithms.[1][2] In 1780, Pierre-Simon Laplace (1749–1827) published a simpler proof of the theorem, which was based on relations between partial derivatives with respect to the variable x and the parameter y.[3][4][5] Charles Hermite (1822–1901) presented the most straightforward proof of the theorem by using contour integration.[6][7][8]

Lagrange's reversion theorem is used to obtain numerical solutions to Kepler's equation.

## Simple proof

We start by writing:

$g(v) = \int \delta(y f(z) - z + x) g(z) (1-y f'(z)) \, dz$

Writing the delta-function as an integral we have:

\begin{align} g(v) & = \int \int \exp(ik[y f(z) - z + x]) g(z) (1-y f'(z)) \, \frac{dk}{2\pi} \, dz \\[10pt] & =\sum_{n=0}^\infty \int \int \frac{(ik y f(z))^n}{n!} g(z) (1-y f'(z)) e^{ik(x-z)}\, \frac{dk}{2\pi} \, dz \\[10pt] & =\sum_{n=0}^\infty \left(\frac{\partial}{\partial x}\right)^n\int \int \frac{(y f(z))^n}{n!} g(z) (1-y f'(z)) e^{ik(x-z)} \, \frac{dk}{2\pi} \, dz \end{align}

The integral over k then gives $\delta(x-z)$ and we have:

\begin{align} g(v) & = \sum_{n=0}^\infty \left(\frac{\partial}{\partial x}\right)^n \left[ \frac{(y f(x))^n}{n!} g(x) (1-y f'(x))\right] \\[10pt] & =\sum_{n=0}^\infty \left(\frac{\partial}{\partial x}\right)^n \left[ \frac{y^n f(x)^n g(x)}{n!} - \frac{y^{n+1}}{(n+1)!}\left\{ (g(x) f(x)^{n+1})' - g'(x) f(x)^{n+1}\right\} \right] \end{align}

Rearranging the sum and cancelling then gives the result:

$g(v)=g(x)+\sum_{k=1}^\infty\frac{y^k}{k!}\left(\frac\partial{\partial x}\right)^{k-1}\left(f(x)^kg'(x)\right)$

## References

1. ^ Lagrange, Joseph Louis (1770) "Nouvelle méthode pour résoudre les équations littérales par le moyen des séries," Mémoires de l'Académie Royale des Sciences et Belles-Lettres de Berlin, vol. 24, pages 251–326. (Available on-line at: http://gdz.sub.uni-goettingen.de/no_cache/dms/load/img/?IDDOC=41070 .)
2. ^ Lagrange, Joseph Louis, Oeuvres, [Paris, 1869], Vol. 2, page 25; Vol. 3, pages 3–73.
3. ^ Laplace, Pierre Simon de (1777) "Mémoire sur l'usage du calcul aux différences partielles dans la théories des suites," Mémoires de l'Académie Royale des Sciences de Paris, vol. , pages 99–122.
4. ^ Laplace, Pierre Simon de, Oeuvres [Paris, 1843], Vol. 9, pages 313–335.
5. ^ Laplace's proof is presented in:
• Goursat, Édouard, A Course in Mathematical Analysis (translated by E.R. Hedrick and O. Dunkel) [N.Y., N.Y.: Dover, 1959], Vol. I, pages 404–405.
6. ^ Hermite, Charles (1865) "Sur quelques développements en série de fonctions de plusieurs variables," Comptes Rendus de l'Académie des Sciences des Paris, vol. 60, pages 1–26.
7. ^ Hermite, Charles, Oeuvres [Paris, 1908], Vol. 2, pages 319–346.
8. ^ Hermite's proof is presented in:
• Goursat, Édouard, A Course in Mathematical Analysis (translated by E. R. Hedrick and O. Dunkel) [N.Y., N.Y.: Dover, 1959], Vol. II, Part 1, pages 106–107.
• Whittaker, E.T. and G.N. Watson, A Course of Modern Analysis, 4th ed. [Cambridge, England: Cambridge University Press, 1962] pages 132–133.