1+2+3+...+n = ?

If you want to know the answer click "Proof and Pictures" link on this page. A clue on how to prove it is given at minute 5:17 of the video stream.

The video is the recording of the presentation given by Professor James Robert Brown on November 3rd as a public lecture organized by the Perimeter Institute about which I talked here.

Note that to see the video you need at least Windows Media Player 9 and a broadband connection of course :)