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21908 = 225477

21908 has 6 divisors (see below), whose sum is σ = 38346. Its totient is φ = 10952.

The previous prime is 21893. The next prime is 21911. The reversal of 21908 is 80912.

It can be written as a sum of positive squares in only one way, i.e., 21904 + 4 = 148^2 + 2^2 .

It is a self number, because there is not a number n which added to its sum of digits gives 21908.

It is a congruent number.

It is an unprimeable number.

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 as a sum of consecutive naturals, namely, 2735 + ... + 2742.

It is an arithmetic number, because the mean of its divisors is an integer number (6391).

221908 is an apocalyptic number.

It is an amenable number.

21908 is a deficient number, since it is larger than the sum of its proper divisors (16438).

21908 is a wasteful number, since it uses less digits than its factorization.

21908 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5481 (or 5479 counting only the distinct ones).

The product of its (nonzero) digits is 144, while the sum is 20.

The square root of 21908 is about 148.0135128966. The cubic root of 21908 is about 27.9812800041.

It can be divided in two parts, 21 and 908, that added together give a palindrome (929).

The spelling of 21908 in words is "twenty-one thousand, nine hundred eight".

Divisors: 1 2 4 5477 10954 21908