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

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Let f(z)=z∏j=1m(z−tj)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 f−1(z)f^{-1}(z) be its inverse branch at 00, whose Taylor series is given by

f−1(z)=∑n=1∞(−n)n−1n!(−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⁡(2πi∑jljϕj−1)(1−∑jϕj)1−∑jϕj∏j(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

(1−∑jλj)(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.

References

Primary source

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

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