21856 has 12 divisors (see below), whose sum is σ = 43092. Its totient is φ = 10912.

The previous prime is 21851. The next prime is 21859. The reversal of 21856 is 65812.

It is a happy number.

21856 is nontrivially palindromic in base 5.

It is an alternating number because its digits alternate between even and odd.

It is a zygodrome in base 5.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21851) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 310 + ... + 373.

It is an arithmetic number, because the mean of its divisors is an integer number (3591).

2²¹⁸⁵⁶ is an apocalyptic number.

21856 is the 94-th centered pentagonal number.

It is an amenable number.

21856 is a deficient number, since it is larger than the sum of its proper divisors (21236).

21856 is an equidigital number, since it uses as much as digits as its factorization.

21856 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 693 (or 685 counting only the distinct ones).

The product of its digits is 480, while the sum is 22.

The square root of 21856 is about 147.8377489006. The cubic root of 21856 is about 27.9591240295.

Subtracting 21856 from its reverse (65812), we obtain a triangular number (43956 = T₂₉₆).

The spelling of 21856 in words is "twenty-one thousand, eight hundred fifty-six".