| Not really a mystery, but more of a "crime drama" in which a former math professor gets two offers he can't refuse: one from a crime boss who wants to hire him as his accountant and another from the police who want him to spy on the crime boss.
The protagonist lost his job as a math professor after becoming interested in gambling and misusing some research fund and ends up working as a short order cook at a fast food restaurant. He also has developed a mental disorder which prevents him from being able to do math anymore. There are many mathematical metaphors (none so good that I think they need to be repeated here) and a surreal dream sequence in which guns fire mathematical symbols.
This short story appears in a collection called Best American mystery Stories of 2002, but I personally cannot see what would qualify it as one. | 
