21979 has 4 divisors (see below), whose sum is σ = 22720. Its totient is φ = 21240.

The previous prime is 21977. The next prime is 21991. The reversal of 21979 is 97912.

21979 = 58² + 59² + ... + 63².

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

It is a cyclic number.

It is not a de Polignac number, because 21979 - 2¹ = 21977 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21977) 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, 324 + ... + 385.

It is an arithmetic number, because the mean of its divisors is an integer number (5680).

2²¹⁹⁷⁹ is an apocalyptic number.

21979 is a deficient number, since it is larger than the sum of its proper divisors (741).

21979 is an equidigital number, since it uses as much as digits as its factorization.

21979 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 740.

The product of its digits is 1134, while the sum is 28.

The square root of 21979 is about 148.2531618550. The cubic root of 21979 is about 28.0114748886.

It can be divided in two parts, 21 and 979, that added together give a cube (1000 = 10³).

The spelling of 21979 in words is "twenty-one thousand, nine hundred seventy-nine".