Numbers / 643

The Number 643

643 is prime: only 1 and 643 divide it evenly; the next prime after 643 is 647. 643's proper divisors total just 1, 642 short of 643, the deficient case most of 643's neighbors share too. 643 has a digit sum of 13; for 643, the digital root works out to 4. 643 shares no factor with 2-12; 643's own divisors start beyond that range. 643 is 1010000011 in binary and 283 in hex; as a Roman numeral it's DCXLIII.