## Tuesday, 14 August 2012

### Sweet 16

The birthday paradox is very funny (well: geek-funny, at least). You may think that it's pretty unlikely that someone you know shares your birthday. In fact, if you know a large enough number of people, the probability of this circumstance goes to 1.

And even if you only have 10 friends, there's still a probability of nearly 12% that one of them was born on the same day as you. Even more strikingly, if you have as little as 23 friends the probability that two of them share a birthday is 50%!

In my case (I turn 16-ish today), this is fully respected. I do have one friend (Russell) who has his birthday today as well. Moreover, my friend Lorenzo has his on the 15th (that's close enough, and actually at one point, when I was living in Boston, we celebrated both our birthdays at the same time $-$ it was already the 15th in Italy, but still the 14th in the US).

Other very close friends do share their birthday with me: among them, Guido Castelnuovo and Mila Kunis(OK: perhaps I don't seem them as much, Guido having lived in the 19th centrury and Mila being busy in Hollywood. But that doesn't make us less friendly...).