Andrei Bely's modernist novel "Petersburg," first published in 1913, is considered a pinnacle of the Symbolist movement. Nabokov famously ranked it as one of the four greatest masterpieces of 20th-century prose. The author's father, Bugaev, was an influential mathematician and for 12 years served as the president of the Moscow Mathematical Society; he was also a source of inspiration for one of the main characters in the son's novel. While the philosophical views and political leanings of the mathematicians surrounding Bely, and their impact on "Petersburg," have been a topic of recent academic interest, there has not yet been a direct investigation of the surprisingly frequent and sophisticated mathematical passages in the book itself. We attempt here to rectify this gap in the scholarly literature, and in doing so find a rich tapestry of mathematical ideas and allusions.