Properties of the Number 639

Six Hundred Thirty-Nine

Basics

Value: 638 → 639 → 640

Parity: odd

Prime: No

Previous Prime: 631

Next Prime: 641

Digit Sum: 18

Digital Root: 9

Palindrome: No

Factorization: 3 ² × 71

Divisors: 1, 3, 9, 71, 213, 639

Polygonal

Triangular: No

Square: No

Pentagonal: No

Hexagonal: No

Heptagonal: No

Tetrahedral: No

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Representations

Binary: 1001111111

Octal: 1177

Duodecimal: 453

Hexadecimal: 27f

Square: 408321

Square Root: 25.278449319529077

Classification

Fibonacci: No

Bell Number: No

Factorial Number: No

Regular Number: No

Perfect Number: No

Special

Kaprekar Constant: No

Munchausen Constant: No

Armstrong Number: No

Kaprekar Number: No

Catalan Number: No

Vampire Number: No

Taxicab Number: No

Super Prime: No

Friedman Number: No

Fermat Number: No

Cullen Number: No

OEIS Sequences

Sum of the first n primes. A7504

a(n) = ∑_k=0..n binomial(n,k)² * binomial(2·k,k). A2893

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. A117817

Average of twin prime pairs (A014574), divided by 2. Equivalently, 2·a(n)-1 and 2·a(n)+1 are primes. A40040

Number of partitions of n whose reciprocal sum is an integer. A58360

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

Solution of the complementary equation a(n) = a(n-1) + a(n-2) + n·b(n-2), where a(0) = 1, a(1) = 2, b(0) = 3, and (a(n)) and (b(n)) are increasing complementary sequences. A296284

Numbers whose sum of digits is 18. A235228

Number of partitions of n such that 2*(least part) >= greatest part. A237824

Twice-partitioned numbers where the first partition is strict and the latter partitions are constant. A279786