who wrote the episode "he ain't heavy"?
CREATE TABLE "table1_27913766_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);
SELECT "written_by" FROM "table1_27913766_1" WHERE "title"='\"He Ain''t Heavy\"';