What was the lowest city population for the district of rajanpur district with a area of 6 km squared and a serial number less than 29?
CREATE TABLE "punjab" (
  "serial_no" real,
  "district" text,
  "headquartered_city" text,
  "city_population_2009" real,
  "city_area_km_2" real
);
SELECT MIN("city_population_2009") FROM "punjab" WHERE "city_area_km_2"=6 AND "district"='rajanpur district' AND "serial_no"<29;