Fatto generale: 21403 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21403 is a boring number.
Trivia: 21403 is an unremarkable number.
Data storica: August 6th is the day in 1926 that Harry Houdini performs his greatest feat, spending 91 minutes underwater in a sealed tank before escaping.