Fatto generale: 17921 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 17921 is an unremarkable number.
Trivia: 17921 is an unremarkable number.
Data storica: January 23rd is the day in 1570 that James Stewart, 1st Earl of Moray, regent for the infant King James VI of Scotland, is assassinated by firearm, the first recorded instance of such.