Log-convexity and log-concavity of hypergeometric-like functions

Abstract

We find sufficient conditions for log-convexity and log-concavity for the functions of the forms $a\mapsto \sum f_k(a{)}_k{x}^k$, $a\mapsto \sum f_k\Gamma (a+k)x^k$ and $a\mapsto \sum f_k{x}^k/(a{)}_k$. The most useful examples of such functions are generalized hypergeometric functions. In particular, we generalize the Turán inequality for the confluent hypergeometric function recently proved by Barnard, Gordy and Richards and log-convexity results for the same function recently proved by Baricz. Besides, we establish a reverse inequality which complements naturally the inequality of Barnard, Gordy and Richards. Similar results are established for the Gauss and the generalized hypergeometric functions. A conjecture about monotonicity of a quotient of products of confluent hypergeometric functions is made.

13 pages, no figures

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