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Some Extensions of W. Gautschi's Inequalities for the Gamma Function
D. Kershaw
Mathematics of Computation
Vol. 41, No. 164 (Oct., 1983), pp. 607611
Published by: American Mathematical Society
DOI: 10.2307/2007697
Stable URL: http://www.jstor.org/stable/2007697
Page Count: 5
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Abstract
It has been shown by W. Gautschi that if $0 < s < 1$, then for $x \geqslant 1$ $$x^{1  s} < \frac{\Gamma(x + 1)}{\Gamma(x + s)} < \exp[(1  s)\psi(x + 1)].$$ The following closer bounds are proved: $$\exp[(1  s)\psi(x + s^{1/2})] < \frac{\Gamma(x + 1)}{\Gamma(x + s)} < \exp\bigg[(1  s)\psi\bigg(x + \frac{s + 1}{2}\bigg)\bigg]$$ and $$\bigg[x + \frac{s}{2}\bigg]^{1  s} < \frac{\Gamma(x + 1)}{\Gamma(x + s)} < \bigg[x  \frac{1}{2} + \bigg(s + \frac{1}{4}\bigg)^{1/2}\bigg]^{1  s}.$$ These are compared with each other and with inequalities given by T. Erber and J. D. Kečkić and P. M. Vasić.
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Mathematics of Computation © 1983 American Mathematical Society