21688 has 8 divisors (see below), whose sum is σ = 40680. Its totient is φ = 10840.

The previous prime is 21683. The next prime is 21701. The reversal of 21688 is 88612.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21683) by changing a digit.

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 as a sum of consecutive naturals, namely, 1348 + ... + 1363.

It is an arithmetic number, because the mean of its divisors is an integer number (5085).

2²¹⁶⁸⁸ is an apocalyptic number.

It is an amenable number.

21688 is a deficient number, since it is larger than the sum of its proper divisors (18992).

21688 is a wasteful number, since it uses less digits than its factorization.

21688 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2717 (or 2713 counting only the distinct ones).

The product of its digits is 768, while the sum is 25.

The square root of 21688 is about 147.2684623400. The cubic root of 21688 is about 27.8873021101.

The spelling of 21688 in words is "twenty-one thousand, six hundred eighty-eight".