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?

The joke is that

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

is the factorial function. So

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