Leopoldstadt, the play by Tom Stoppard, has a mathematician in it, Ludwig, and some mathematics.

People always are asking Ludwig to examine their child, who they think might be a prodigy. Ludwig always asks the children, 'what is the sum of all the numbers 1-20?' and/or '1-100?'. If they answer quickly, he asks for the sum of 1-100; if they don't answer that quickly, they fail the test. How does Ludwig know?

(If you look it up then you not only aren't a prodigy, you're a dumbass.)