21587981 has 4 divisors (see below), whose sum is σ = 21602172. Its totient is φ = 21573792.

The previous prime is 21587963. The next prime is 21587987. The reversal of 21587981 is 18978512.

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

It can be written as a sum of positive squares in 2 ways, for example, as 13942756 + 7645225 = 3734^2 + 2765^2 .

It is a cyclic number.

It is not a de Polignac number, because 21587981 - 2¹⁴ = 21571597 is a prime.

It is a super-2 number, since 2×21587981² = 932081847312722, 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 (21587987) 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, 4496 + ... + 7961.

It is an arithmetic number, because the mean of its divisors is an integer number (5400543).

Almost surely, 2^21587981 is an apocalyptic number.

It is an amenable number.

21587981 is a deficient number, since it is larger than the sum of its proper divisors (14191).

21587981 is a wasteful number, since it uses less digits than its factorization.

21587981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 14190.

The product of its digits is 40320, while the sum is 41.

The square root of 21587981 is about 4646.2867970025. The cubic root of 21587981 is about 278.4436655661.

The spelling of 21587981 in words is "twenty-one million, five hundred eighty-seven thousand, nine hundred eighty-one".