Fatto generale: 1645 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1645 is a boring number.
Trivia: 1645 is an unremarkable number.
Data storica: July 2nd is the day in 1964 that U.S. President Lyndon B. Johnson signs the Civil Rights Act of 1964 meant to prohibit segregation in public places.