I think there is a typo at least in the e-book version.
When the professor explains how to use two adjacent triangles to get the sum of the natural numbers from 1 to 4, the book gives the following diagram, which makes no sense:
4 4 4 4
+ + + +
1 1 1 1
-------
5 5 5 5
whereas my German edition has the diagram
1 2 3 4
+ + + +
4 3 2 1
-------
5 5 5 5
which fits better to the figure of the two triangles (taking the sum in each row):
I would think it would be a lot more terrifying to find notes covered in blood, with one saying you can only remember things from the last 80 minutes. Memento vibes.
I’m not sure I 100% got the meaning of why Root was crying.
ママが博士を信用しなかったからだよ。博士に僕の世話は任せられないんじゃないかって、少しでも疑ったことが許せないんだ。
Is he broken up because she doubted the professor’s ability to take care of him, even though in the end it seemed to be warranted?
Was it warranted, though? Yes, 博士 overreacted in a major way, but clearly out of affection. I think Root is growing attached to him-- like a father figure-- and doesn’t want his mother looking down on his father figure.
The ending of the chapter is so, so sweet. What a charming book.