Fatto matematico: 21437 is an unremarkable number.
Trivia: 21437 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: September 9th is the day in 1969 that Allegheny Airlines Flight 853 DC-9 collides in flight with a Piper PA-28 and crashes near Fairland, Indiana.