21581 has 4 divisors (see below), whose sum is σ = 24672. Its totient is φ = 18492.

The previous prime is 21577. The next prime is 21587. The reversal of 21581 is 18512.

21581 = T₇₅ + T₇₆ + ... + T₈₁.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21581 - 2² = 21577 is a prime.

It is a super-2 number, since 2×21581² = 931479122, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21587) 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, 1535 + ... + 1548.

It is an arithmetic number, because the mean of its divisors is an integer number (6168).

2²¹⁵⁸¹ is an apocalyptic number.

It is an amenable number.

21581 is a deficient number, since it is larger than the sum of its proper divisors (3091).

21581 is an equidigital number, since it uses as much as digits as its factorization.

21581 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3090.

The product of its digits is 80, while the sum is 17.

The square root of 21581 is about 146.9047310334. The cubic root of 21581 is about 27.8413648481.

The spelling of 21581 in words is "twenty-one thousand, five hundred eighty-one".