21697 has 4 divisors (see below), whose sum is σ = 23380. Its totient is φ = 20016.

The previous prime is 21683. The next prime is 21701. The reversal of 21697 is 79612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 7056 + 14641 = 84^2 + 121^2 .

It is a cyclic number.

It is not a de Polignac number, because 21697 - 2⁷ = 21569 is a prime.

It is a Duffinian number.

It is a plaindrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (21617) 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, 822 + ... + 847.

It is an arithmetic number, because the mean of its divisors is an integer number (5845).

2²¹⁶⁹⁷ is an apocalyptic number.

It is an amenable number.

21697 is a deficient number, since it is larger than the sum of its proper divisors (1683).

21697 is a wasteful number, since it uses less digits than its factorization.

21697 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1682.

The product of its digits is 756, while the sum is 25.

The square root of 21697 is about 147.2990156111. The cubic root of 21697 is about 27.8911590972.

It can be divided in two parts, 216 and 97, that added together give a palindrome (313).

The spelling of 21697 in words is "twenty-one thousand, six hundred ninety-seven".