21956 has 12 divisors (see below), whose sum is σ = 42000. Its totient is φ = 9960.

The previous prime is 21943. The next prime is 21961. The reversal of 21956 is 65912.

It is a plaindrome in base 15.

It is an unprimeable number.

21956 is an untouchable number, because it is not equal to the sum of proper divisors of any 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 3 ways as a sum of consecutive naturals, for example, 206 + ... + 293.

It is an arithmetic number, because the mean of its divisors is an integer number (3500).

2²¹⁹⁵⁶ is an apocalyptic number.

It is an amenable number.

21956 is a deficient number, since it is larger than the sum of its proper divisors (20044).

21956 is a wasteful number, since it uses less digits than its factorization.

21956 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 514 (or 512 counting only the distinct ones).

The product of its digits is 540, while the sum is 23.

The square root of 21956 is about 148.1755715359. The cubic root of 21956 is about 28.0017005770.

Subtracting 21956 from its reverse (65912), we obtain a triangular number (43956 = T₂₉₆).

The spelling of 21956 in words is "twenty-one thousand, nine hundred fifty-six".