Fatto generale: 21839 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21839 is a boring number.
Trivia: 21839 is a boring number.
Data storica: October 16th is the day in 456 that Magister militum Ricimer defeats Emperor Avitus at Piacenza and becomes master of the Western Roman Empire.