643

643 is a prime number.

641 and 643 form a twin prime pair.

643 is the largest prime factor of 123456: 643(64³) = 123456.

643 = (3⁸ + 8³)/(3 + 8).

A record 643 millimeters of rain fell in Sydney, Australia, in the month of June 1950.

Source: Prime Curios!