The explicit mathematical references all occur in the first half of the book that focuses on Bassam Aramin. They begin with a reasonably precise mathematical definition of its title:

(For a more rigorous definition, see this entry at Wolfram's Math World.)

Math connects to the plot of the novel when Aramin is an inmate at an Israeli prison and his guard, who happens to be a graduate student studying mathematics, notices that his prisoner ID number is mathematically interesting:

Cleverly, the next section is numbered 220 and that is where the author defines amicable numbers and explains why the numbers 220 and 284 are paired in this way, "as if those different things of which they are comprised can somehow recognize one another".

Of course, the analogy here is that Hertzl and Bassam are like those amicable numbers, different but somehow amicably connected:

Even Hertz's topic of research seems to have a clever double meaning here, referring both to the mathematical integration of harmonic functions and also possibly to the idea of two cultures living together peacefully in an integrated society.

Please don't misunderstand me: math is only one tiny piece of this novel that focuses on many small details to provide some context to the tragic deaths of the two daughters which, sadly, are not fiction. I presume that Hertzl and the entire mathematical component of the story was the creation of the author. (Please correct me if I'm mistaken.) If so, then I would argue that the author is creatively using mathematical fiction as a way to add some beauty and meaning to some of the ugly truths of the real world. Despite the choice of a mathematical title, mathematics is not a particularly important component and the only mathematician is a relatively minor character. But, the little bit that is there is used very effectively.