Fatto generale: 15817 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 15817 is an uninteresting number.
Trivia: 15817 is a boring number.
Data storica: April 21st is the day in 1894 that Norway formally adopts the Krag-Jørgensen rifle as the main arm of its armed forces, a weapon that would remain in service for almost 50 years.