Fatto generale: 56539 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 56539 is an unremarkable number.
Trivia: 56539 is an unremarkable number.
Data storica: October 18th is the day in 1860 that the Second Opium War finally ends at the Convention of Peking with the ratification of the Treaty of Tientsin, an unequal treaty.