21691 has 4 divisors (see below), whose sum is σ = 22000. Its totient is φ = 21384.

The previous prime is 21683. The next prime is 21701. The reversal of 21691 is 19612.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21691 - 2³ = 21683 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (21601) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10 + ... + 208.

It is an arithmetic number, because the mean of its divisors is an integer number (5500).

2²¹⁶⁹¹ is an apocalyptic number.

21691 is a deficient number, since it is larger than the sum of its proper divisors (309).

21691 is a wasteful number, since it uses less digits than its factorization.

21691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 308.

The product of its digits is 108, while the sum is 19.

The square root of 21691 is about 147.2786474680. The cubic root of 21691 is about 27.8885878910. Note that the first 3 decimals are identical.

The spelling of 21691 in words is "twenty-one thousand, six hundred ninety-one".