Fait mathématique: 7923 is a value of n for which n and 2n together use each digit 1-9 exactly once.
Culture générale: 7923 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: September 9th is the day in 1922 that the Greco-Turkish War of 1919-1922 effectively ends with Turkish victory over the Greeks in Smyrna.