21556 has 12 divisors (see below), whose sum is σ = 40068. Its totient is φ = 10112.

The previous prime is 21529. The next prime is 21557. The reversal of 21556 is 65512.

It is a happy number.

21556 is an esthetic number in base 16, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in 2 ways, for example, as 8100 + 13456 = 90^2 + 116^2 .

It is a super-2 number, since 2×21556² = 929322272, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21557) by changing a digit.

21556 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 91 + ... + 226.

It is an arithmetic number, because the mean of its divisors is an integer number (3339).

2²¹⁵⁵⁶ is an apocalyptic number.

It is an amenable number.

21556 is a deficient number, since it is larger than the sum of its proper divisors (18512).

21556 is a wasteful number, since it uses less digits than its factorization.

21556 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 338 (or 336 counting only the distinct ones).

The product of its digits is 300, while the sum is 19.

The square root of 21556 is about 146.8196172179. The cubic root of 21556 is about 27.8306099702.

Subtracting 21556 from its reverse (65512), we obtain a triangular number (43956 = T₂₉₆).

The spelling of 21556 in words is "twenty-one thousand, five hundred fifty-six".