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Someone told me this, but I dont get it. $$\Gamma (Happy Birthday + 1)$$ Why is this the way to greet a mathematician on his birthday with an excitement?

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3  
the gamma function extends $n!$, $\Gamma(n+1)=n!$ – yoyo Aug 25 '13 at 17:52
up vote 9 down vote accepted

The joke is that

$$\Gamma(n + 1) = n!$$

is the factorial function. So

$$\Gamma(\text{Happy Birthday} + 1) = \text{Happy Birthday}!$$

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5  
since when is Happy Birthday a natural number? – Dominic Michaelis Aug 25 '13 at 17:52
5  
@DominicMichaelis Well, one does generally celebrate on natural-number # of years... – Steven Stadnicki Aug 25 '13 at 17:54
5  
@DominicMichaelis I suppose that by the time one learns of the gamma function, one has stopped celebrating half birthdays ;) – user61527 Aug 25 '13 at 17:56

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