Number of the day: 669

Properties of the number 669:

669 = 3 × 223 is semiprime and squarefree.
669 has 2 distinct prime factors, 4 divisors, 7 antidivisors and 444 totatives.
669 has a semiprime digit sum 21 in base 10.
669 has a Fibonacci digit sum 21 in base 10.
669 has a triangular digit sum 21 in base 10.
669 = 335² - 334² = 113² - 110² is the difference of 2 nonnegative squares in 2 ways.
669 is the sum of 2 positive triangular numbers.
669 is the difference of 2 positive pentagonal numbers in 1 way.
669 = 10² + 13² + 20² is the sum of 3 positive squares.
669² is the sum of 3 positive squares.
669 is a proper divisor of 1783² - 1.
669 = '6' + '69' is the concatenation of 2 semiprime numbers.
669 is an emirpimes in (at least) the following bases: 3, 5, 11, 14, 17, 21, 22, 24, 25, 26, 27, 28, 31, 35, 37, 43, 44, 45, 48, 51, 53, 59, 60, 75, 78, 79, 85, 89, 92, 93, 94, and 99.
669 is palindromic in (at least) the following bases: -18, and -23.
669 in base 6 = 3033 and consists of only the digits '0' and '3'.
669 consists of only the digits '6' and '9'.
669 in base 11 = 559 and consists of only the digits '5' and '9'.
669 in base 25 = 11j and consists of only the digits '1' and 'j'.

The number 669 belongs to the following On-Line Encyclopedia of Integer Sequences (OEIS) sequences (among others):

Sequence numbers and descriptions below are taken from OEIS.
A001743: Every digit contains at least one loop (version 1).
A001748: 3 * primes.
A025147: Number of partitions of n into distinct parts >= 2.
A028505: Number of primes <= 100*n.
A051227: Bernoulli number B_{2n} has denominator 42.
A059729: Carryless squares n X n base 10.
A161345: Numbers k whose largest divisor <= sqrt(k) is 3.
A192065: Expansion of prod(k>=1, Q(x^k)^k ) where Q(x)=prod(k>=1, 1+x^k ).
A265072: Coordination sequence for (3,3,5) tiling of hyperbolic plane.
A298035: Coordination sequence of Dual(3.12.12) tiling with respect to a trivalent node.