Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: 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?

### Accurate SQL ###

SELECT MIN(population_per_km²__2009_) FROM table_name_21 WHERE province = "solomon islands" AND area__km²_ < 28 OFFSET 400