math_fact: 4691 is a value of n for which n and 8n together use each digit 1-9 exactly once.
trivia: 4691 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
historical_date: November 3rd is the day in 1333 that the River Arno flooding causing massive damage in Florence as recorded by the Florentine chronicler Giovanni Villani.