How to greet a mathematician on his birthday with an excitement

Someone told me this, but I dont get it. I hope this is on topic. Otherwise please tell me and I will delete this question. $$\Gamma (Happy Birthday + 1)$$ Why is this the way to greet a mathematician on his birthday with an excitement?

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

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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since when is Happy Birthday a natural number? –  Dominic Michaelis Aug 25 '13 at 17:52
@DominicMichaelis Well, one does generally celebrate on natural-number # of years... –  Steven Stadnicki Aug 25 '13 at 17:54
@DominicMichaelis I suppose that by the time one learns of the gamma function, one has stopped celebrating half birthdays ;) –  T. Bongers Aug 25 '13 at 17:56
Thanks. Haha I get it now. –  Leo Aug 25 '13 at 18:05