21905 has 8 divisors (see below), whose sum is σ = 28392. Its totient is φ = 16128.

The previous prime is 21893. The next prime is 21911. The reversal of 21905 is 50912.

It is a Cunningham number, because it is equal to 148²+1.

It can be written as a sum of positive squares in 4 ways, for example, as 1 + 21904 = 1^2 + 148^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21905 - 2⁶ = 21841 is a prime.

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 in 7 ways as a sum of consecutive naturals, for example, 104 + ... + 233.

It is an arithmetic number, because the mean of its divisors is an integer number (3549).

2²¹⁹⁰⁵ is an apocalyptic number.

It is an amenable number.

21905 is a deficient number, since it is larger than the sum of its proper divisors (6487).

21905 is a wasteful number, since it uses less digits than its factorization.

21905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 355.

The product of its (nonzero) digits is 90, while the sum is 17.

The square root of 21905 is about 148.0033783398. The cubic root of 21905 is about 27.9800027284.

The spelling of 21905 in words is "twenty-one thousand, nine hundred five".