Well, I wrote about something broad, example) people who apply to applied mathematics, applied sciences mathematics, mathematics, etc. I would just list in general, mathematics on the essay. Not sure if it makes a big difference but it seems to be universal for all the colleges.
edit: I'm not sure how you would relate poli sci and econ..
edit2: for some schools, the personal statement is not a very important factor. so you can focus your statements on schools such as UCB, UCLA, UCM, and UCI? I think those would be the only schools that really consider your essay... I heard other schools only use it if you are a borderline candidate.