CREATE TABLE table_21728 ("Series #" FLOAT, "Episode #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "U.S. viewers  ( million ) " TEXT, "Original airdate" TEXT, "Production Code" TEXT)

Who wrote the episode 'The Cold Turkey', which was viewed by 3.73 million viewers?
SELECT "Written by" FROM table_21728 WHERE "U.S. viewers (million)" = '3.73'
SELECT "Written by" FROM "table_21728" WHERE "U.S. viewers (million)" = '3.73'