Book about a young genius, with a math duel

I read this book a long time ago, and all I remember about it is a single scene. A young genius (I'll refer to him as "he", I think it was a boy, but it might have been a girl, not 100% sure) is in a room with a bunch of old professors, maybe taking an entrance exam? He tells one of the math professors that he made a mistake in one of his papers, and begins writing equations to prove it on the blackboard. The professor interjects a few times, then can't stand it any longer and picks up a chalk to write his own equations on the other side of the blackboard. They silently duel with equations like this, writing on the walls when they run out of blackboard, and eventually meet up at the other end of the room, having filled the walls with equations. Both come to a final conclusion: one gets "x=0", and the other gets "x=∞".

The professor says something like "well obviously we still have some slight disagreements, but very interesting, young man". Then he tells the other professors that they should get this boy into the university, and when they ask him if he means a scholarship, he says no, he should be given a chair and made a professor.

That is all I remember. Any ideas?