Zhao's log-concavity conjecture for the Fennessey-Larcombe-French sequence

Let {Vn}n0\{V_n\}_{n\geq 0} be the Fennessey-Larcombe-French sequence, defined by V0=1V_0=1, V1=8V_1=8, and

n(n+1)2Vn+1=8n(3n2+5n+1)Vn128(n1)(n+1)2Vn1,n1.n(n+1)^2 V_{n+1}=8n(3n^2+5n+1)V_n-128(n-1)(n+1)^2V_{n-1},\qquad n\geq 1.

A sequence {an}\{a_n\} is log-concave when an2an1an+1a_n^2\geq a_{n-1}a_{n+1} for every relevant index. Zhao's conjecture. The Fennessey-Larcombe-French sequence {Vn}n1\{V_n\}_{n\geq 1} is log-concave; that is,

Vn2Vn1Vn+1(n2).V_n^2\geq V_{n-1}V_{n+1}\qquad(n\geq 2).

The conjecture concerns the log-behavior of the sequence arising from the series expansion of the complete elliptic integral of the second kind. The paper records it after proving that {nVn}n1\{nV_n\}_{n\geq 1} and {Vn/(n1)!}n1\{V_n/(n-1)!\}_{n\geq 1} are log-concave; its resolution is not specified in the supplied text.

Sources & referencesView supporting material

Primary source

Arthur L. B. Yang and James J. Y. Zhao, “Log-concavity of the Fennessey-Larcombe-French Sequence”, arXiv:1503.02151 (2015).

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