Table schema:
CREATE TABLE table_name_15 (city_population__2009_ INTEGER, serial_no VARCHAR, city_area_km_2__ VARCHAR, district VARCHAR)

Question: 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?
SQL: SELECT MIN(city_population__2009_) FROM table_name_15 WHERE city_area_km_2__ = 6 AND district = "rajanpur district" AND serial_no < 29