Fatto matematico: 81641 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 81641 is an unremarkable number.
Data storica: July 11th is the day in 1302 that Battle of the Golden Spurs (Guldensporenslag in Dutch) – a coalition around the Flemish cities defeats the king of France's royal army.