# Bürmann-Lagrange series

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Lagrange series

A power series which offers a complete solution to the problem of local inversion of holomorphic functions. In fact, let a function of the complex variable be regular in a neighbourhood of the point , and let and . Then there exists a regular function in some neighbourhood of the point of the -plane which is the inverse to and is such that . Moreover, if is any regular function in a neighbourhood of the point , then the composite function can be expanded in a Bürmann–Lagrange series in a neighbourhood of the point

 (*)

The inverse of the function is obtained by setting .

The expansion (*) follows from Bürmann's theorem [1]: Under the assumptions made above on the holomorphic functions and , the latter function may be represented in a certain domain in the -plane containing in the form

where

Here is a contour in the -plane which encloses the points and , and is such that if is any point inside , then the equation has no roots on or inside other than the simple root .

The expansion (*) for the case was obtained by J.L. Lagrange .

If the derivative has a zero of order at the point , there is the following generalization of the Bürmann–Lagrange series for the multi-valued inverse function [3]:

Another generalization (see, for example, [4]) refers to functions regular in an annulus; instead of the series (*), one obtains a series with positive and negative powers of the difference .

#### References

 [1] H. Bürmann, Mem. Inst. Nat. Sci. Arts. Sci. Math. Phys. , 2 (1799) pp. 13–17 [2a] J.L. Lagrange, Mem. Acad. R. Sci. et Belles-lettres Berlin , 24 (1770) [2b] J.L. Lagrange, "Additions au mémoire sur la résolution des équations numériques" , Oeuvres , 2 , G. Olms (1973) pp. 579–652 [3] A. Hurwitz, R. Courant, "Vorlesungen über allgemeine Funktionentheorie und elliptische Funktionen" , 1 , Springer (1968) pp. Chapt. 7 [4] E.T. Whittaker, G.N. Watson, "A course of modern analysis" , Cambridge Univ. Press (1952) pp. Chapt. 6 [5] A.I. Markushevich, "Theory of functions of a complex variable" , 1 , Chelsea (1977) (Translated from Russian)