I wrote about this some time ago[0] and submitted it here on HN[1], where there was an interesting discussion. That article includes a complete derivation[2] that's simply quoted in this submission.<p>[0] <a href="http://www.solipsys.co.uk/new/TheBirthdayParadox.html?HN-20150513" rel="nofollow">http://www.solipsys.co.uk/new/TheBirthdayParadox.html?HN-201...</a><p>[1] <a href="https://news.ycombinator.com/item?id=4753014" rel="nofollow">https://news.ycombinator.com/item?id=4753014</a><p>[2] <a href="http://www.solipsys.co.uk/new/TheBirthdayParadox.html#toc_name002" rel="nofollow">http://www.solipsys.co.uk/new/TheBirthdayParadox.html#toc_na...</a>