Most of the mathematical content occurs at the beginning of the book when Charlie is working at a facility in Chicago computing trajectories. There he works with other mathematicians and there is room for some mathematical humor (such as their computations to determine which desks are the most desirable). Looking back with the knowledge of what new weapons were being developed at the end of World War II, we can also see that the trajectories themselves are ominous, but Charlie -- who cannot imagine why one would want to detonate a bomb at such a high altitude -- is oblivious to their seriousness.

Also in Chicago, Charlie meets Brenda Dubie whose family runs a music store. Their quirky love story, one in which Brenda herself doesn't quite understand her own feelings for a man who is so physically weak and in which she does not even tell him that she loves him until surprisingly late in their relationship, is certainly a greater focus of the story than mathematics. So, too, is Charlie's moral strength, which Brenda only comes to appreciate later.

When Charlie is transferred to Los Alamos, his work becomes more practical and less mathematical as it is his job to actually build the electronic circuit that will cause the simultaneous detonation of conventional explosives that then causes the nuclear chain reaction. Getting them to explode at the same time is difficult. As he seems to be the only one capable of doing it, Charlie becomes an essential part of the project. Over his frequent objections, everyone begins calling him "Trigger".

Eventually, he does use mathematics and lots of equations to solve the problem of creating a working detonator for the first nuclear bomb, though I could not really tell from the vague description what sort of mathematics he was using.

Throughout all of this, he refers to himself as "just a mathematician", even after the war when he tries being a graduate student in physics. However, the historical figure on whom Charlie was based does not seem to have been a mathematician. So, the math is apparently something that the author has "sprinkled" into this story, like a spice to enhance a dish. In my opinion, it was a success resulting in a book that is worth reading.