Our mathematically-inclined young author, who knows English well, may have been struck by the fact that in English "odd" can refer both to numbers not divisible by 2 and to things and especially people who are strange, who don't fit in. From there he may have elaborated the metaphor of prime numbers (divisible only by themselves and 1) as mathematical misfits, all prime numbers being odd numbers except for 2 (I looked it up). He further develops the idea by refining it to include twin primes, prime numbers that are separated by only one even number e.g. 17 and 19, but become increasingly rare as one counts upwards.
Are you getting the uncomfortable feeling that you're back in high school? Well, you won't if you read this novel. He carries all this off splendidly. Alice and Mattia are followed over the course of years from childhood to adulthood, and they are our twin primes. Both are scarred, literally and emotionally, by early events. Brilliant Mattia actually had a twin, who unlike him was mentally backward. His strong urge to belong, to not be odd, leads him to abandon her one day in the park so that he can go to a party as a "normal" person. The sister disappears forever without a trace, paradoxically leaving Mattia more odd and isolated than ever, as he isn't even paired with his biological twin anymore.
Can Mattia and Alice become two, which is a prime number but not odd? At the height of their relationship, budding photographer Alice has them dress as a married couple and takes a snapshot. But tragically, Giordano implies (correctly I think) that people who come together because of deficiencies or wounds are not coming together on a solid, healthy basis. The metaphor of the twin primes also cues us to the fact that they are necessarily apart. The subtle ending of the book does offer some glimmer, not of optimism, but of the idea that even deeply damaged people can go on, and like the crippled Alice, get up by herself, or like Mattia, see the dawn in a new country.
(In the photo, writer and physicist Paolo Giordano, looking very cute)