How do you prove Gautschi’s inequality for the gamma function?

A few answers here on math.SE have used as an intermediate step the following inequality that is due to Walter Gautschi:

$$x^{1-s} < \frac{\Gamma(x+1)}{\Gamma(x+s)} < (x+1)^{1-s},\qquad x > 0,\; 0 < s < 1$$

Unfortunately, the paper that the DLMF is pointing to is not easily accessible. How might this inequality be proven?

Answer

The strict log-convexity of $\Gamma$ (see the end of this answer) implies that for $0< s <1$,
$$
\Gamma(x+s)<\Gamma(x)^{1-s}\Gamma(x+1)^s=x^{s-1}\Gamma(x+1)\tag{1}
$$
which yields
$$
x^{1-s}<\frac{\Gamma(x+1)}{\Gamma(x+s)}\tag{2}
$$
Again by the strict log-convexity of $\Gamma$,
$$
\Gamma(x+1)<\Gamma(x+s)^s\Gamma(x+s+1)^{1-s}=(x+s)^{1-s}\Gamma(x+s)\tag{3}
$$
which yields
$$
\frac{\Gamma(x+1)}{\Gamma(x+s)}<(x+s)^{1-s}<(x+1)^{1-s}\tag{4}
$$
Combining $(2)$ and $(4)$ yields
$$
x^{1-s}<\frac{\Gamma(x+1)}{\Gamma(x+s)}<(x+1)^{1-s}\tag{5}
$$

Attribution
Source : Link , Question Author : J. M. ain’t a mathematician , Answer Author : Community

Leave a Comment