Fatto generale: 51640 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 51640 is an uninteresting number.
Trivia: 51640 is an uninteresting number.
Data storica: May 20th is the day in 1983 that First publications of the discovery of the HIV virus that causes AIDS in the journal Science by Luc Montagnier.