against
Explanation
This comic is a mathematics joke centered on the concept of the "Erdos number."
In the panel, an older woman (likely a mathematician) says "I once wrote a paper with Paul Erdos." A younger man responds competitively: "Yeah? Well I once wrote a paper AGAINST Paul Erdos."
The caption reads: "It's easy to get an Erdos Number of -1."
The Erdos number is a real concept in mathematics. Paul Erdos (1913-1996) was an extraordinarily prolific Hungarian mathematician who published more papers with more co-authors than almost anyone in history. An Erdos number measures collaborative distance: if you co-authored a paper directly with Erdos, your Erdos number is 1. If you co-authored with someone who co-authored with Erdos, yours is 2, and so on. Having a low Erdos number is a point of pride in academic mathematics.
The joke invents the concept of a negative Erdos number by imagining someone who wrote a paper not "with" but "against" Erdos -- as if academic collaboration were a competitive sport where you could be on the opposing team. The humor lies in treating the Erdos number system, which measures collaborative proximity, as though it has a directional or adversarial dimension. Writing "against" someone is the opposite of collaboration, so it humorously inverts the number to -1.