Fatto generale: 6391 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 6391 is a hexagonal pyramidal number.
Trivia: 6391 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: June 30th is the day in 1905 that Albert Einstein publishes the article On the Electrodynamics of Moving Bodies, in which he introduces special relativity.