Conjecture on the radius of convergence for generalized Lambert W inverse series

From papers

Let f(z)=zj=1m(ztj)pjezf(z)=z\prod_{j=1}^m(z-t_j)^{p_j}e^z, where the tjt_j and pjp_j are nonzero complex numbers, and let f1(z)f^{-1}(z) be its inverse branch at 00, whose Taylor series is given by

f1(z)=n=1(n)n1n!(t1)np1(tm)npmFnzn.f^{-1}(z)=\sum_{n=1}^{\infty}\frac{(-n)^{n-1}}{n!}(-t_1)^{-np_1}\cdots(-t_m)^{-np_m}F_nz^n.

Radius-of-convergence conjecture. There exist integers l1,,lml_1,\ldots,l_m such that the radius of convergence of this series is

R=exp(2πijljϕj1)(1jϕj)1jϕjj(tjpj+ϕj)pj+ϕjpjpjϕjϕj,R=\left|\exp\left(2\pi i\sum_jl_j\phi_j-1\right)\left(1-\sum_j\phi_j\right)^{1-\sum_j\phi_j}\prod_j\left(\frac{t_j}{p_j+\phi_j}\right)^{p_j+\phi_j}p_j^{p_j}\phi_j^{\phi_j}\right|,

where ϕ\phi is one of the solutions of

(1jλj)(pi+λi)+λiti=0.\left(1-\sum_j\lambda_j\right)(p_i+\lambda_i)+\lambda_it_i=0.

The conjecture identifies the convergence radius through the critical points governing the inverse function's analytic continuation. The source gives no resolution, so the status of this formula remains open.

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Sources & referencesView supporting material

Primary source

Paul Castle, “Taylor series for generalized Lambert W functions”, arXiv:1801.09904 (2018).

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