21858 has 8 divisors (see below), whose sum is σ = 43728. Its totient is φ = 7284.

The previous prime is 21851. The next prime is 21859. The reversal of 21858 is 85812.

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

21858 is an admirable number.

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21851) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1816 + ... + 1827.

It is an arithmetic number, because the mean of its divisors is an integer number (5466).

2²¹⁸⁵⁸ is an apocalyptic number.

21858 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (21864).

21858 is a wasteful number, since it uses less digits than its factorization.

21858 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3648.

The product of its digits is 640, while the sum is 24.

The square root of 21858 is about 147.8445129181. The cubic root of 21858 is about 27.9599768318.

It can be divided in two parts, 218 and 58, that added together give a triangular number (276 = T₂₃).

The spelling of 21858 in words is "twenty-one thousand, eight hundred fifty-eight".