Fait général: 21537 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fait mathématique: 21537 is an uninteresting number.
Culture générale: 21537 is a boring number.
Date historique: December 18th is the day in 1898 that Gaston de Chasseloup-Laubat sets the first officially recognized land speed record of 39.245 mph (63.159 km/h) in a Jeantaud electric car.