math_fact: 4392 is a value of n for which n and 4n together use each digit 1-9 exactly once.
trivia: 4392 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
historical_date: January 9th is the day in 475 that Byzantine Emperor Zeno is forced to flee his capital at Constantinople, and his general, Basiliscus gains control of the empire.