Numbers / 661

The Number 661

661 is prime: only 1 and 661 divide it evenly; the next prime after 661 is 673. 661's proper divisors total just 1, 660 short of 661, the deficient case most of 661's neighbors share too. 661 has a digit sum of 13; for 661, the digital root works out to 4. 661 shares no factor with 2-12; 661's own divisors start beyond that range. 661 is 1010010101 in binary and 295 in hex; as a Roman numeral it's DCLXI.