PhD is in math, not grammar, and there is a possibility English isn't his first language.

As an aside, I looked at the author's thesis because I thought normally (at least at my school) they include a "Vita" that gives the candidates educational experience. This thesis didn't have that, I'm wondering now if it's not a common thing.

Thanks for pointing that out, I'll adjust the title.

I'm guessing the PhD wasn't in English.

Or maybe English isn't their first language.