21656 has 8 divisors (see below), whose sum is σ = 40620. Its totient is φ = 10824.

The previous prime is 21649. The next prime is 21661. The reversal of 21656 is 65612.

21656 = 29² + 30² + ... + 44².

It is a tau number, because it is divible by the number of its divisors (8).

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

It is a congruent number.

It is an unprimeable number.

21656 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 as a sum of consecutive naturals, namely, 1346 + ... + 1361.

2²¹⁶⁵⁶ is an apocalyptic number.

It is an amenable number.

21656 is a deficient number, since it is larger than the sum of its proper divisors (18964).

21656 is a wasteful number, since it uses less digits than its factorization.

21656 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2713 (or 2709 counting only the distinct ones).

The product of its digits is 360, while the sum is 20.

The square root of 21656 is about 147.1597771132. The cubic root of 21656 is about 27.8735797302.

Subtracting 21656 from its reverse (65612), we obtain a triangular number (43956 = T₂₉₆).

It can be divided in two parts, 216 and 56, that added together give a palindrome (272).

The spelling of 21656 in words is "twenty-one thousand, six hundred fifty-six".