The murder victim's brother-in-law is a high school math teacher. Nero Wolfe believes this to be relevant at one point, even quoting some mathematical history from an encyclopedia.

I first read Death of a Doxy in the early '70s. Just picked it up by chance in a bus station. It was my first Nero Wolfe story. I liked it... until the end when I felt that Saul turned up magically from nowhere with the solution......Mathematics, itself, was pretty much non-existant in the book. The character taught Math and there was a clue in the name of an ancient Mathematician-- which eludes me at the moment [Note: It was Thales of Milton -alex] -- but the purpose of Mathematics was to provide that clue. Mathematics, itself, contributed nothing to the story.

I really like the Nero Wolfe books. I find it refreshing that Stout (the author) occasionally drops some math/arithmetic as well as basic science in the stories and expects that to be no unusual burden on the reader. He even uses words that chase us ordinary mortals to the nearest dictionary. In other words, your intelligence is not insulted and that is a good thing.