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Gruber P. Convex and Discrete Geometry

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18 <strong>Convex</strong> Functions<br />

(ii) Let x > 0. Then<br />

Ɣ(x + 1) =<br />

�<br />

+∞<br />

0<br />

t x e −t dt = lim<br />

�s<br />

� x −t<br />

= lim − t e<br />

s→+∞<br />

� �s + x 0<br />

= x lim<br />

�s<br />

s→+∞<br />

0<br />

t<br />

s→+∞<br />

0<br />

x e −t dt<br />

�s<br />

t x−1 e −t dt �<br />

t x−1 e −t dt = xƔ(x).<br />

(iii) Let x, y > 0 <strong>and</strong> 0 0,<br />

are precisely the functions of the form g(x) = Ɣ(x)h(x) for x > 0, where h : R + →<br />

R is any function with period 1 <strong>and</strong> h(1) = 1. Among this large family of functions,<br />

Ɣ is singled out by the property of logarithmic convexity, as shown by Bohr <strong>and</strong><br />

Mollerup [136]:<br />

Theorem 1.11. Let g : R + → R + be a function having the Properties (i)–(iii) of<br />

Theorem 1.10. Then g = Ɣ on R + .<br />

Proof. Properties (i) <strong>and</strong> (ii) imply that

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