What is the episode number of the episode broadcast on 21 September 2001?
CREATE TABLE "episode_information" (
  "episode_number" text,
  "broadcast_date" text,
  "title" text,
  "written_by" text,
  "viewership_millions" real
);
SELECT "episode_number" FROM "episode_information" WHERE "broadcast_date"='21 september 2001';