At Saturday's Stanford Celebration of Mind, one of the speakers (DEK?), keeping with the Centennial theme, exhibited Ernie Parker's 10x10 orthogonal Latin square, as featured in what was probably Gardner's first Scientific American cover story. Most of us have heard that the nonexistence of a 4n+2 square was one of Euler's few unlucky conjectures. But the speaker added that Julius Petersen, the father of Graph Theory, had actually published a (bogus) proof of Euler's incorrect dismissal of 4n+2. This is impressive scholarship--not in Wikipedia? --rwg