"Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes), volume 8½ by Aoyagi Aito. It's a murder mystery with a bit of recreational math.
The story opens with a song that contains three linear algebra equations, which is a brilliant nerd sniping move in that the kind of people who would read this novel series are also the type who would try to solve them mentally. This opening deserved the extra attention since it outlined the three murders that followed, each carefully arranged to reference a verse in this song. This song-and-murder construction reminds me of Agatha Christie's "And Then There Were None", but this is a very different novel, and the real secret encoded in this song actually solves a different mystery in the end.
After the opening, the story proceeded in two parallel tracks: Ryuunosuke narrates the "crane" side where the murders took place, while Azusa narrates the "turtle" side with seemly irrelevant events. Nagisa seem to be missing for most of this volume until she literally popped out of nowhere, and that's the moment where the two tracks join to start the great reveal process. Appearance of Nagisa is also where the fun with math begins, starting with the history behind Dudeney dissection and ending with a certain French mathematician that ties back to the opening song. This has been a great story as far as murder mysteries go, with a carefully interleaved timeline.
Despite the strength in the mystery aspects, this is still a series about math. As author explains in the afterwords, the premise of "a village where using formulas leads to death" came about from a common experience in elementary and high schools where students are not allowed to use formulas that they have not been taught yet. Despite the inconvenience in lacking formulas, people still find ways to practice math in their own intuitive ways. This book might be a very good murder mystery, but the author definitely wanted people to enjoy the math in it.
Math has been enjoyable as usual. Expect the return of the usual terrorist plot in volume 9.
This is the second volume to have a single long story instead of short chapters, the previous one was volume 3½.
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