"Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes) volume 5 by Aoyagi Aito. This is the volume where author probably finally realized that the terrorists were too likable.
On the math side, topics covered in this volume includes magic squares, pigeonhole principle, and conic sections. Pigeonhole principle was especially surprising to see, since it's one of those very obvious theorems that does not seem to have many applications. Turns out, it can be used to prove statements such as "any 5 random distinct integers will contain at least a pair whose difference is a multiple of 4". Other than that, the use of boxes in kanji characters to specify magic squares was an interesting twist, which was totally outdone by the transition to magic cubes later. The math topics were all very entertaining, consistent with previous volumes.
On the story side, the author has been trying to make the terrorist plot more relevant. Notice how almost every terrorist character up to volume 4 had very likable traits, and it's only due to their excessive passion about math that led them into joining this terrorist group. At some point, readers should start to wonder if this series needed any terrorist plot at all -- it might as well be Nagisa the wandering detective who solves math mysteries. Indeed, one of the chapters for this volume was exactly that, and it worked quite well.
But following the reorganization near the end of volume 4, the terrorists have turned violent again, with the new terrorist faction hunting the more moderate member of the old faction. It's an attempt to bring back the sense of urgency that were in the earlier volumes. Either the novel series will end soon, or the threat will be forgotten and the next volume will continue in the same leisurely pace as usual, most likely the latter. I just hope that future math villains will be just as quirky.
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