What is the lowest density of a town with a 13,708 2011 population census ?
CREATE TABLE "lg_as" (
  "local_government_area" text,
  "principal_town" text,
  "land_area_km" real,
  "population_2011_census" real,
  "density" real
);
SELECT MIN("density") FROM "lg_as" WHERE "population_2011_census"='13,708';