MathOverflow will be down for maintenance for approximately 3 hours, starting Monday evening (06/24/2013) at approximately 9:00 PM Eastern time (UTC-4).
    Post Made Community Wiki by S. Carnahan
show/hide this revision's text 2 added 237 characters in body

Cantor's diagonal argument.

The warm-up could be an equally beautiful proof, namely that the rationals are countable.

show/hide this revision's text 1

Cantor's diagonal argument.