21686 has 8 divisors (see below), whose sum is σ = 37200. Its totient is φ = 9288.

The previous prime is 21683. The next prime is 21701. The reversal of 21686 is 68612.

It is a sphenic number, since it is the product of 3 distinct primes.

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 polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 761 + ... + 788.

It is an arithmetic number, because the mean of its divisors is an integer number (4650).

2²¹⁶⁸⁶ is an apocalyptic number.

21686 is a deficient number, since it is larger than the sum of its proper divisors (15514).

21686 is a wasteful number, since it uses less digits than its factorization.

21686 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1558.

The product of its digits is 576, while the sum is 23.

The square root of 21686 is about 147.2616718634. The cubic root of 21686 is about 27.8864448569.

It can be divided in two parts, 21 and 686, that added together give a palindrome (707).

The spelling of 21686 in words is "twenty-one thousand, six hundred eighty-six".