CREATE TABLE table_name_81 (population_density__per_km²_ VARCHAR(255), name VARCHAR(255), capital VARCHAR(255))
What is Population density (per km²), when Name is Antigua and Barbuda, and when Capital is St. John's?
    
SQL: SELECT population_density__per_km²_ FROM table_name_81 WHERE name = "antigua and barbuda" AND capital = "st. john's"