Fatto matematico: 721 is the smallest number which can be written as the difference of 2 cubes in 2 ways.
Trivia: 721 is an unremarkable number.
Data storica: December 21st is the day in 1967 that Louis Washkansky, the first man to undergo a heart transplant, dies in Cape Town, South Africa, after living for 18 days after the transplant.