"Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes) volume 4 by Aoyagi Aito. In contrast with the heavy number theory stuff from 3½, we are back to the lighter recreational math topics for this volume: probability, paper folding, square roots, and linear algebra with one unknown. Easy topics presented in a lighthearted way.
The probability bits were a highlight for this novel, especially in its treatment of the Monty Hall problem. Details of this problem are explained in this chapter in a more lively way than most online sources on this topic: in a game show setting with a host named Monkey Hall, and real menacing monkeys. Similar to previous volumes, the characters managed to find themselves in a contrived scenario that happened to fit a particular math problem, facing terrorists who happened to be passionate (but clumsy) mathematicians.
Skip a bit to the last chapter, these characters ended up in a musical while three linear equations. The equation bits are very straightforward, with an interesting twist later in the chapter that unified all earlier equations. These were all well and fine, but the musical bits was even more contrived than the game show. Imagine "The Sound of Music" with names of mathematicians as lyrics... honestly this didn't work out so well. The author later said in the afterword: "it was always my dream to make a musical with math". The scenarios were really totally random, but at least math bits were still very good.
My personal favorite from this volume was when Nagisa nerd-sniped[1] the terrorist with this question: "how many nets can be folded in 3 different ways to form a box?"[2]. Given the terrorists' background, I am surprised that nerd-sniping doesn't happen more often.
Overall, still a very enjoyable series, even if the scenarios get more and more surreal.
[1] https://xkcd.com/356/
[2] Example net that can be folded in 2 different ways (same example was included in this book): http://www.jaist.ac.jp/~uehara/etc/origami/nets/index-e.html
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