"Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes), volume 3½ by Aoyagi Aito. Being a series about mathematics, I guess it wouldn't be right if all volume numbers are strictly integers, although this particular volume focuses on one of the greatest integer problems in history.
If you had guessed the topic was Fermat's Last Theorem, you would be mostly right. I was totally looking forward to this novel series coming around to cover Fermat's Last Theorem, and here is one long murder mystery dedicated to it. Fermat's Last Theorem: the novel. Oh my! Unfortunately, there is currently no easy proof to Fermat's Last Theorem, definitely not one that would fit a novel of this size. That said, this novel covered much of the history around Fermat and it did gave an outline of the proof for the n=4 special case. As an introductory pointer to a very interesting topic, this novel is doing a great job.
Saying that this novel is dedicated to Fermat's Last Theorem would only be mostly right. The key to solving the final mystery actually has more Pascal than Fermat. Besides Fermat and Pascal, it also included few other topics in recreational mathematics such as the balance puzzle and the 15-puzzle, and a very curious math orientated version of mahjong (definitely viable as a real game, but likely not one that would be commercially successful). These are more consistent with the difficulty level in previous volumes, and are very fun to read as usual.
In other unrelated topics, this is the novel where readers discover the origin of the notebook that Nagisa carries around with her all the time. "What! Unexpectedly, the mysterious origin of Nagisa's calculation notes has just been unceremoniously revealed." Also, there is a great illustration of an unsolvable 15-puzzle next to the corresponding text, but you will need to turn to the appendix to find a much smaller illustration of Nagisa in a swimsuit. The lack of attention and interest to anything not math related appears to be a running joke for this series. I am sure the author made the narrator a non-mathematician intentionally, otherwise there wouldn't be much story elements left at all.
All in all, the format has changed slightly to fit the 1/2 designation, but it's still the same great math-centric novel. Back to the usual numbering for the next volume.
Previous (2015-02-19): "Hamamura Nagisa no Keisan Nooto" volume 3 by Aoyagi Aito
Next (2015-03-26): "Good Eater" by Araki Shin