The Solitude of Prime Numbers (original title: La solitudine dei numeri primi) is a novel by the Italian author Paolo Giordano, published in 2008.
Although the setting is never mentioned explicitly, references to Gran Madre Church, the Basilica of Superga, the Maria Ausiliatrice hospital, and to Fraitève and Monte Fraiteve identify the locale as Turin.
Mattia Balossino is a gifted and intelligent child, unlike his twin sister Michela, who has a intellectual disability.
Upon his return to the park a few hours later, Michela has disappeared, perhaps drowned in a nearby pond, and is never found despite a police search and investigation.
Both attend a university, where Mattia studies mathematics, and where Alice drops-out to pursue her passion for photography.
Mattia, meanwhile, completes his degree and receives an offer to teach at a university in Northern Europe (probably in Norway).
While he is debating whether to leave Italy and take the job, Mattia finally tells Alice about Michela, and the two kiss for the first time.
Fabio wants a child, but Alice, who hasn't menstruated for years because of anorexia, cannot get pregnant and refuses to change her eating habits.