Fait général: 20411 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fait mathématique: 20411 is an unremarkable number.
Culture générale: 20411 is a boring number.
Date historique: November 18th is the day in 1421 that a seawall at the Zuiderzee dike in the Netherlands breaks, flooding 72 villages and killing about 10,000 people.