Although in reality the Collatz Conjecture remains unresolved, in Billot's novel the problem was famously solved by Etienne Thèseus, who figured out the solution while he fought for France in Algeria during the 1960's. Thèseus is a cruel man, abusing both his students and the women in his life, who enjoys great success and acclaim. But, his reputation is put at risk by a young mathematician who may have a better proof of the conjecture and who just happens to be the grandson of an Algerian mathematician that Thèseus personally tortured during the war.
The Collatz Conjecture is known by many names, including the "Syracuse Problem". This name derives not from the ancient Syracuse but from Syracuse University in New York State. Nevertheless, this name seems to have inspired a mythical theme for Billot's novel. (A woman named Ariadne plays an important role for this "Theseus", as it did for the one who escaped from the Minotaur.)
As far as I know, this book has only appeared in French. (Consequently, I have not actually read it. The summary above is based only on reviews and comments that I was able to find on the internet. I hope I have not misrepresented anything.) I do try to watch for new works of mathematical fiction, but this one somehow evaded my notice. I am grateful to Vijay Fafat who brought it to my attention in 2021.
