21715981 has 8 divisors (see below), whose sum is σ = 24872000. Its totient is φ = 18573408.

The previous prime is 21715951. The next prime is 21715997. The reversal of 21715981 is 18951712.

It is a happy number.

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

It is not a de Polignac number, because 21715981 - 2⁷ = 21715853 is a prime.

It is a super-2 number, since 2×21715981² = 943167661584722, 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 (21715921) 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, 385 + ... + 6601.

It is an arithmetic number, because the mean of its divisors is an integer number (3109000).

Almost surely, 2^21715981 is an apocalyptic number.

It is an amenable number.

21715981 is a deficient number, since it is larger than the sum of its proper divisors (3156019).

21715981 is an equidigital number, since it uses as much as digits as its factorization.

21715981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6723.

The product of its digits is 5040, while the sum is 34.

The square root of 21715981 is about 4660.0408796490. The cubic root of 21715981 is about 278.9928998763.

The spelling of 21715981 in words is "twenty-one million, seven hundred fifteen thousand, nine hundred eighty-one".