Questions tagged [lagrange-inversion]

Use of the Lagrange–Bürmann formula, which gives the Taylor series expansion of the inverse function of an analytic function.

The Lagrange inversion theorem expresses the Taylor series of analytic function’s inverse or its composition with another function $g(x)$. The general statement of theorem is:

Let $y=f(x)$ and $y_0=f(x_0)$ where $f’(x_0)\ne0$, then
$$g(x)=g(x_0)+\sum_{k=1}^\infty\frac{(y-y_0)^k}{k!}\left\{\frac{d^{k-1}}{dx^{k-1}}\left[g’(x)\left(\frac{x-x_0}{f(x)-y_0}\right)^k\right]\right\}_{x=x_0}$$

68 questions
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Functional inverse of $z=1+w+\cdots+w^{n-1}$

Migrated to MO. I am interested in the functional inverse of $$ z=1+w+\cdots+w^{n-1},\quad w\geq0,\ n>1. $$ This function is strictly increasing on $w\geq0$ and thus admits an inverse. My attempt: By Lagrange's theorem we may write the…
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Generating function of $\binom{3n}{n}$

Wolfram alpha tells me the ordinary generating function of the sequence $\{\binom{3n}{n}\}$ is given by $$\sum_{n} \binom{3n}{n} x^n = \frac{2\cos[\frac{1}{3}\sin^{-1}(\frac{3\sqrt{3}\sqrt{x}}{2})]}{\sqrt{4-27x}}$$ How do I prove this?
user17982
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What does the $q$-Catalan Numbers count?

I had completed a paper describing the $q$-Catalan numbers, which is the $q$-analog of the Catalan numbers. The $n$-th Catalan numbers can be represented by: $$C_n=\frac{1}{n+1}{2n \choose n}$$ and with the recurrence…
user67258
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Why is the bizarre $f^{-1}(y)=y+\sum_{n=1}^\infty\frac{1}{n!}\frac{d^{n-1}}{dy^{n-1}}[y-f(y)]^n$ equivalent to the Lagrange inversion formula?

$\newcommand{\d}{\mathrm{d}}$EDIT: In the nontrivial example $f(x)=x-\frac{1}{4}x^3$ and using either of the two series to produce a result for $f^{-1}(3/4)$, I find that $(\ast)$ and $(1)$ produce the same result. It would appear that they actually…
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Multivariate Lagrange inversion with powers

Let the formal power series $\phi_1,\dots,\phi_m$ in the variables $x_1,\dots,x_m$ be defined by \begin{equation} \phi_i(x_1,\dots,x_m)=x_i\rho_i(\phi_1,\dots,\phi_m),\qquad i = 1,\dots,m \end{equation} for some formal series…
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Convert $\frac1b\sum_{n=1}^\infty\frac{(b e^a)^n}{n!}B_{n-1}(an)$ to integral using $B_n(x)=\frac{n!}{2\pi i}\oint\frac{e^{x(e^t-1)}}{t^{n+1}}dt$

$\def\B{\operatorname B}$ In How to solve $x^{y^z}=z$ A solution uses Bell polynomials $\B_n(x)$ $$e^{ae^{bz}}=z=1+\frac1b\sum_{n=1}^\infty \frac{(ae^b)^n}{nn!}\B_n(b n)=\frac1b\sum_{n=1}^\infty\frac{(b e^a)^n}{n!}\B_{n-1}(an)\tag1$$ A contour…
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Explicit series expansion for inverse of $e^{-x}\left(\frac{x^2}2+x+1\right)$

Intro: Remember that the W-Lambert function has the following series expansion: $$xe^x=y\implies x=\text W(y)=\sum_{n=1}^\infty\frac{(-n)^{n-1}y^n}{n!},|y|<\frac1e$$ so how about a series expansion for inverse of…
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Lagrange Inversion Theorem Proof

Note: throughout this question, I'll be using the following notation convention: $$f^{(n)}(x)=\frac{d^nf}{dx^n}(x)$$ I was browsing through Wikipedia and even MSE's related questions searching for a proof for the Lagrange Inversion Theorem. I'm…
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Series expansion for $2$ real root branches of the equation $x^p-x+t=0$

A well known infinite series for the solution of the equation $x^p-x+t=0$ for $p>1$ is: $$x=\sum_{k=0}^{\infty}\binom{pk}{k}\frac{t^{(p-1)k+1}}{(p-1)k+1}$$ Which is well defined in the region $0\le t\le (p-1)p^{-p/(p-1)}$. This series when $t=0$…
Thinh Dinh
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Inverting $\frac{\xi}{2}(1+\tanh(\xi))=\lambda$ using the Lagrange-Burmann Theorem

For my quantum mechanics homework, I developed the transcendental equation $\frac{\xi}{2}(1+\tanh(\xi))$ for the well-posedness of symmetric potential formed from two delta functions. The professor encourages us to use a numerical tool to solve the…
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Solving $x + 3^x < 4$ analytically

I am solving the problems in Michael Spivak's Calculus book. In the Prologue chapter, there is the following problem: $$x + 3^x < 4$$ I can solve this using graphical means, but not analytically. Is it possible to do the latter? I can arrive at…
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Numerical inverse of a function

How to approximate the inverse of the function below? $$f(x) = \frac34 x - \frac 12\sin(2x) + \frac 1{16} \sin(4x)$$ The goal is to get $x$ values (range $[0, \pi]$) from values of $f$. The function doesn't seem to have analytical inverse, so I…
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Evaluating $\left.\frac{d^{n-1}}{dt^{n-1}}e^{(1-n)t}\ln^n(e^t+1)\right|_0$ to solve $x^{ax}=x+1$

In trying to solve $x^{ax}=x+1$, like for the Foias Ewing constant equation $z^{z+1}=(z+1)^z$, one uses Lagrange reversion which seems to only converge in the following form after letting $x=e^t$: $$x^{ax}=x+1\iff t=\frac1ae^{-t}\ln(e^t+1)\implies…
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How helpful is $f^{-1}(z)=\frac1{2\pi i}\oint\ln(1-\frac z{f(w)})dw$, or the method to find it, in deriving integral representations of $f^{-1}(z)$?

$\DeclareMathOperator \erf{erf}$ Wolfram Alpha gives the following $\erf^{-1}(z)$ series: $$\sum_{n=1}^\infty\frac{z^n}{2\pi n}\int_0^{2\pi}e^{it}\erf(e^{it})^{-n}dt$$ which can be derived via Lagrange inversion and the inverse Z transform. $\erf$…
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Simplified expression for the nth derivative of the n+1th power of a function

$$\frac{d^n}{dx^n}(f(x))^n \text{ and similar expressions}$$ I was trying to work out the Taylor series for the solution to the general cubic using Lagrange Inversion (out of curiosity: I was wondering which of the roots the series would give for…
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