Fatto generale: 21431 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21431 is an uninteresting number.
Trivia: 21431 is an unremarkable number.
Data storica: September 3rd is the day in 1874 that the congress of the state of México elevates Naucalpan to the category of Villa, with the title of "Villa de Juárez".