name the sorority a.k.a. "sigmas" that was founded in the university of southern california.
CREATE TABLE "asian_american" (
  "letters" text,
  "organization" text,
  "nickname" text,
  "founding_date" text,
  "founding_university" text,
  "type" text
);
SELECT "organization" FROM "asian_american" WHERE "founding_university"='University of Southern California' AND "type"='Sorority' AND "nickname"='\"Sigmas\"';