2012-09-22
Explanation
This is a single-panel comic with a caption underneath. A bearded man sitting at a cafe tells someone across the table: "I'm making a you substitution." The caption below reads: "It's hard getting dumped by a mathematician."
The joke is a play on the mathematical technique called "u-substitution" (also known as integration by substitution), a common method in calculus where you replace a complicated expression with a simpler variable "u" to make an integral easier to solve. The mathematician is using this as a breakup line, saying he is making a "you substitution" -- meaning he is replacing "you" (his partner) with someone else, just as in calculus you replace one variable with another. The pun works because "u" and "you" sound the same.
The votey panel extends the joke: the mathematician cheerfully says "Now you're 'dx'!" In u-substitution, when you substitute u for some expression, the original differential (like dx) gets transformed and often becomes a leftover piece of the integral. By calling his ex "dx," the mathematician is saying they have been reduced to an insignificant remainder -- the discarded part of the equation after the important substitution has been made.