General fact: 21473 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 21473 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 21473 is a boring number.
Historical date: October 15th is the day in 2003 that the Staten Island Ferry boat Andrew J. Barberi runs into a pier at the St. George Ferry Terminal in Staten Island, killing 11 people and injuring 43.