644

644 = 2² x 7 x 23.

644 is a Perrin number.

644 is 212212 in base 3 and 2552 in base 6.

644 is a number made up of only semiprime digits (4, 6, and 9).

644 is the sum of five fourth powers: 644 = 5⁴ + 2⁴ + 1⁴ + 1⁴ + 1⁴.

The USS Lewis and Clark (SSBN-644) was a ballistic missile submarine in the U.S. Navy.

Source: What's Special About This Number?