21906 has 12 divisors (see below), whose sum is σ = 47502. Its totient is φ = 7296.

The previous prime is 21893. The next prime is 21911. The reversal of 21906 is 60912.

It can be written as a sum of positive squares in only one way, i.e., 19881 + 2025 = 141^2 + 45^2 .

It is a Harshad number since it is a multiple of its sum of digits (18), and also a Moran number because the ratio is a prime number: 1217 = 21906 / (2 + 1 + 9 + 0 + 6).

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 in 5 ways as a sum of consecutive naturals, for example, 591 + ... + 626.

2²¹⁹⁰⁶ is an apocalyptic number.

21906 is an abundant number, since it is smaller than the sum of its proper divisors (25596).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21906 is a wasteful number, since it uses less digits than its factorization.

21906 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1225 (or 1222 counting only the distinct ones).

The product of its (nonzero) digits is 108, while the sum is 18.

The square root of 21906 is about 148.0067566025. The cubic root of 21906 is about 27.9804284999.

The spelling of 21906 in words is "twenty-one thousand, nine hundred six".