ABSTRACT
There is an anecdote about a very well-known professor of mathematics which, if it is not true, deserves to be. It relates that, while addressing a learned audience, he wrote a mathematical statement on the board, saying 'This, of course, is obvious.' Looking at it again, he said 'At least, I think it is obvious.' Growing more doubtful, he said 'Excuse me,' and taking pencil and paper, was absent from the room for about twenty minutes. He returned beaming and said triumphantly 'Yes, gentlemen, it is obvious.'
