Hecho matemático: 645 is the largest n for which 1+2+3+ ... +n = 1^{2}+2^{2}+3^{2}+ ... +k^{2} for some k.
Trivia: 645 is an unremarkable number.
Fecha histórica: October 6th is the day in 1976 that New Premier Hua Guofeng orders the arrest of the Gang of Four and associates and ends the Cultural Revolution in the People's Republic of China.