Fatto generale: 17521 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 17521 is an uninteresting number.
Trivia: 17521 is an unremarkable number.
Data storica: December 20th is the day in 1988 that the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances is signed in Vienna.