21558 has 8 divisors (see below), whose sum is σ = 43128. Its totient is φ = 7184.

The previous prime is 21557. The next prime is 21559. The reversal of 21558 is 85512.

It is an interprime number because it is at equal distance from previous prime (21557) and next prime (21559).

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

21558 is an admirable number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21557) 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, 1791 + ... + 1802.

It is an arithmetic number, because the mean of its divisors is an integer number (5391).

2²¹⁵⁵⁸ is an apocalyptic number.

21558 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 (21564).

21558 is a wasteful number, since it uses less digits than its factorization.

21558 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3598.

The product of its digits is 400, while the sum is 21.

The square root of 21558 is about 146.8264281388. The cubic root of 21558 is about 27.8314706663.

The spelling of 21558 in words is "twenty-one thousand, five hundred fifty-eight".