"Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes) volume 8 by Aoyagi Aito. This volume features imaginary numbers, amicable numbers, how to compute square roots, and how to move a sofa through a hallway.
The first chapter includes some history of Arabic numerals and al-Khwarizmi's method for computing square roots, which is further explained in more detail in the appendix. This is a very useful bit of knowledge for the days when you need to compute things by hand, but the real highlight of the chapter was this one weird trick in determining the ratio of sand to air in a bottle that is only partially cylindrical. The application is perhaps oddly specific, but somehow this novel series managed to work it into part of the storyline in a very fun away.
Another neat trick included in this volume was the moving sofa problem, which I thought was rather neat when I first heard about it[1], but thought this is yet another oddly specific problem. Nonetheless, this novel series managed to make it a key element in solving a particular murder mystery, thereby deducing that the murderer is (of course) a math-orientated terrorist. The best part about this series is being able to weave practical and interesting math knowledge into scenarios like these, and this volume continue to maintain that spirit throughout. It's only in a series like this where you can find a punchline like "that's amazing, he managed to make the criminal confess without using any math!"
Math aside, there were some recurring characters who made some surprising appearances, and seems like there is slight change in this terrorist business, but mostly the world is the same as before. The cliffhanger geography puzzle from previous volume was promptly resolved in the prologue, while this volume ends with a new puzzle that can be interpreted in many ways. Looking forward to another series of oddly specific scenarios in the next volume.
[1] https://news.ycombinator.com/item?id=13296502
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