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21681 = 331173
BaseRepresentation
bin101010010110001
31002202000
411102301
51143211
6244213
7120132
oct52261
932660
1021681
1115320
1210669
139b3a
147c89
156656
hex54b1

21681 has 16 divisors (see below), whose sum is σ = 35520. Its totient is φ = 12960.

The previous prime is 21673. The next prime is 21683. The reversal of 21681 is 18612.

It is not a de Polignac number, because 21681 - 23 = 21673 is a prime.

It is a super-2 number, since 2×216812 = 940131522, which contains 22 as substring.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21681.

It is not an unprimeable number, because it can be changed into a prime (21683) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 261 + ... + 333.

It is an arithmetic number, because the mean of its divisors is an integer number (2220).

221681 is an apocalyptic number.

It is an amenable number.

21681 is a deficient number, since it is larger than the sum of its proper divisors (13839).

21681 is a wasteful number, since it uses less digits than its factorization.

21681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 93 (or 87 counting only the distinct ones).

The product of its digits is 96, while the sum is 18.

The square root of 21681 is about 147.2446943017. The cubic root of 21681 is about 27.8843014935.

The spelling of 21681 in words is "twenty-one thousand, six hundred eighty-one".

Divisors: 1 3 9 11 27 33 73 99 219 297 657 803 1971 2409 7227 21681