One for the mathematicians amongst us. But not exclusively: there is a crime to be solved and we follow the clues and the suspects as we would in any other mystery. The perpetrator is very lucky that so many of the other major characters understand so much about number sequences.
There is even a quiz for the reader, entwined within the narrative. I tried so hard to work it out but to no avail. Later, I found the answer online and almost laughed out loud at the simplicity and the clues I had missed.