Fatto generale: 21547 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21547 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 21547 is a boring number.
Data storica: December 28th is the day in 1612 that Galileo Galilei becomes the first astronomer to observe the planet Neptune, although he mistakenly catalogued it as a fixed star.