Symbolism, Imagery, Allegory
Shmoop doesn't need to do much for this one because Christopher pretty much lays it out for us:
Prime numbers are what is left when you have taken all the patterns away. I think prime numbers are like life. They are very logical but you could never work out the rules, even if you spent all your time thinking about them. (19.6)
Prime numbers are like life. Period. Do you agree? Why do you think Christopher chose to use prime numbers to number his chapters?