# How to greet a mathematician on his birthday with an excitement

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

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