Published in 1966.   Orrie Cather, one of Wolfe's operatives, has been secretly seeing a wealthy man's kept mistress at her secret love nest.  He is arrested when she turns up dead.   Orrie is the only one of Wolfe's operatives to have the plot of two Stout books turn on his actions: Death of a Doxy and Stout's final work, A Family Affair.

This is one of three Wolfe stories using clues related to relatively arcane mathematical trivia: "The Zero Clue" in Three Men Out, And Be a Villain, and Death of a Doxy.
synopsis source: www.nerowolfe.org