I think that's stuff from the periodic table on her butt, but I can't really decipher it. There's no Ha element, so I assume that's some kind of hydrogen (H, not sure what the a means. I've forgotten most of the stuff from chemistry class over the years). Ga is Gallium, Na is Sodium and the last one seems to be I, Iodine. No, I have no idea why these seemingly unrelated elements are on her butt.

Popebug said:

No, I have no idea why these seemingly unrelated elements are on her butt.

It's actually Japanese abbreviation for the series, haganai

usr234 said:

It's actually Japanese abbreviation for the series, haganai

Ah. That explains it.