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.
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