Fatto generale: 81991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 81991 is an uninteresting number.
Trivia: 81991 is a boring number.
Data storica: June 25th is the day in 1944 that the final page of the comic Krazy Kat was published, exactly two months after its author George Herriman died.