### Properties of the number 639:

639 = 3^{2}× 71 is the 523

^{th}composite number and is not squarefree.

639 has 2 distinct prime factors, 6 divisors, 5 antidivisors and 420 totatives.

639 = 320

^{2}- 319

^{2}= 108

^{2}- 105

^{2}= 40

^{2}- 31

^{2}is the difference of 2 nonnegative squares in 3 ways.

639 is the sum of 2 positive triangular numbers.

639 is the difference of 2 positive pentagonal numbers in 1 way.

639 is not the sum of 3 positive squares.

639

^{2}is the sum of 3 positive squares.

639 is a divisor of 1277

^{2}- 1.

639 = '6' + '39' is the concatenation of 2 semiprime numbers.

639 is palindromic in (at least) the following bases: 22, and 70.

639 in base 8 = 1177 and consists of only the digits '1' and '7'.

639 in base 14 = 339 and consists of only the digits '3' and '9'.

639 in base 21 = 199 and consists of only the digits '1' and '9'.

639 in base 22 = 171 and consists of only the digits '1' and '7'.

### The number 639 belongs to the following On-Line Encyclopedia of Integer Sequences (OEIS) sequences (among others):

Sequence numbers and descriptions below are taken from OEIS.A002893: a(n) = Sum_{k=0..n} binomial(n,k)^2 * binomial(2k,k).

A006884: In the `3x+1' problem, these values for the starting value set new records for highest point of trajectory before reaching 1.

A007504: Sum of first n primes.

A040040: Average of twin prime pairs (A014574), divided by 2. Equivalently, 2*a(n)-1 and 2*a(n)+1 are primes.

A060980: |First digit - second digit + third digit - fourth digit ...| = 12.

A087097: Lunar primes (formerly called dismal primes) (cf. A087062).

A117817: Let T_n be the infinite sequence formed by starting with 1 and repeatedly reversing the digits and adding n to get the next term. If T_n eventually reaches a cycle, sequence gives length of that cycle, otherwise -1.

A121029: Multiples of 9 containing a 9 in their decimal representation.

A187220: Gullwing sequence (see Comments lines for precise definition).

A235228: Numbers whose sum of digits is 18.

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