I got it too, and I think Eliezer was vastly under confident in his estimate of the number of people who would get it: We have a very mathy community around here.
In any case, I am glad Eliezer ended up including it. HPMOR is good enough to make me actually laugh out loud at least once every chapter, and the bit about the hydras was chapter 100's contribution.
I found it jarring. Either the characters were making jokes at a very uncharacteristic time, or just saying things that make no sense; either way this derails them. It seems to be a general problem in fanfic.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 99, 100, and 101. The previous thread is at nearly 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system.
Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: