Table schema:
CREATE TABLE table_name_21 (population_per_km²__2009_ INTEGER, province VARCHAR, area__km²_ VARCHAR)

Question: What is the lowest Population per km² (2009) that has a Solomon Islands province, with an Area (km²) smaller than 28,400?
SQL: SELECT MIN(population_per_km²__2009_) FROM table_name_21 WHERE province = "solomon islands" AND area__km²_ < 28 OFFSET 400