21718981 has 4 divisors (see below), whose sum is σ = 21747852. Its totient is φ = 21690112.

The previous prime is 21718973. The next prime is 21718993. The reversal of 21718981 is 18981712.

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 476100 + 21242881 = 690^2 + 4609^2 .

It is a cyclic number.

It is not a de Polignac number, because 21718981 - 2³ = 21718973 is a prime.

It is a super-2 number, since 2×21718981² = 943428271356722, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21718981.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21718381) 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, 13276 + ... + 14821.

It is an arithmetic number, because the mean of its divisors is an integer number (5436963).

Almost surely, 2^21718981 is an apocalyptic number.

It is an amenable number.

21718981 is a deficient number, since it is larger than the sum of its proper divisors (28871).

21718981 is an equidigital number, since it uses as much as digits as its factorization.

21718981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 28870.

The product of its digits is 8064, while the sum is 37.

The square root of 21718981 is about 4660.3627541212. The cubic root of 21718981 is about 279.0057466389.

The spelling of 21718981 in words is "twenty-one million, seven hundred eighteen thousand, nine hundred eighty-one".