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21805 = 57289
BaseRepresentation
bin101010100101101
31002220121
411110231
51144210
6244541
7120400
oct52455
932817
1021805
1115423
1210751
139c04
147d37
1566da
hex552d

21805 has 12 divisors (see below), whose sum is σ = 30780. Its totient is φ = 14784.

The previous prime is 21803. The next prime is 21817. The reversal of 21805 is 50812.

It is a happy number.

It can be written as a sum of positive squares in 2 ways, for example, as 5929 + 15876 = 77^2 + 126^2 .

It is not a de Polignac number, because 21805 - 21 = 21803 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21803) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 201 + ... + 289.

It is an arithmetic number, because the mean of its divisors is an integer number (2565).

221805 is an apocalyptic number.

It is an amenable number.

21805 is a deficient number, since it is larger than the sum of its proper divisors (8975).

21805 is an equidigital number, since it uses as much as digits as its factorization.

21805 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 108 (or 101 counting only the distinct ones).

The product of its (nonzero) digits is 80, while the sum is 16.

The square root of 21805 is about 147.6651617681. The cubic root of 21805 is about 27.9373599697.

The spelling of 21805 in words is "twenty-one thousand, eight hundred five".

Divisors: 1 5 7 35 49 89 245 445 623 3115 4361 21805