21679 has 8 divisors (see below), whose sum is σ = 26240. Its totient is φ = 17496.

The previous prime is 21673. The next prime is 21683. The reversal of 21679 is 97612.

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

It is a 7-Lehmer number, since φ(21679) divides (21679-1)⁷.

It is a cyclic number.

It is not a de Polignac number, because 21679 - 2⁵ = 21647 is a prime.

It is a Duffinian number.

Its product of digits (756) is a multiple of the sum of its prime divisors (189).

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21673) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 52 + ... + 214.

It is an arithmetic number, because the mean of its divisors is an integer number (3280).

2²¹⁶⁷⁹ is an apocalyptic number.

21679 is a deficient number, since it is larger than the sum of its proper divisors (4561).

21679 is a wasteful number, since it uses less digits than its factorization.

21679 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 189.

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

The square root of 21679 is about 147.2379027289. The cubic root of 21679 is about 27.8834440558.

It can be divided in two parts, 2167 and 9, that multiplied together give a triangular number (19503 = T₁₉₇).

The spelling of 21679 in words is "twenty-one thousand, six hundred seventy-nine".