Asymptotic expansions for the incomplete gamma function in the transition regions

Gergo Nemes, Adri Olde Daalhuis

Research output: Contribution to journalArticlepeer-review

Abstract

We construct asymptotic expansions for the normalised incomplete gamma function Q(a, z) = Γ(a, z)/Γ(a) that are valid in the transition regions, including the case za, and have simple polynomial coefficients. For Bessel functions, these type of expansions are well known, but for the normalised incomplete gamma function they were missing from the literature. A detailed historical overview is included. We also derive an asymptotic expansion for the corresponding inverse problem, which has importance in probability theory and mathematical statistics. The coefficients in this expansion are again simple polynomials, and therefore its implementation is straightforward. As a byproduct, we give the first complete asymptotic expansion as a → −∞ of the unique negative zero of the regularised incomplete gamma function γ* (a, x).
Original languageEnglish
Pages (from-to)1805-1827
Number of pages21
JournalMathematics of computation
Volume88
Issue number318
Early online date8 Nov 2018
DOIs
Publication statusE-pub ahead of print - 8 Nov 2018

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