21668981 has 4 divisors (see below), whose sum is σ = 21739872. Its totient is φ = 21598092.

The previous prime is 21668963. The next prime is 21669007. The reversal of 21668981 is 18986612.

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 21668981 - 2⁶ = 21668917 is a prime.

It is a super-2 number, since 2×21668981² = 939089475156722, which contains 22 as substring.

It is a Duffinian 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 (21668681) 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, 34985 + ... + 35598.

It is an arithmetic number, because the mean of its divisors is an integer number (5434968).

Almost surely, 2^21668981 is an apocalyptic number.

It is an amenable number.

21668981 is a deficient number, since it is larger than the sum of its proper divisors (70891).

21668981 is an equidigital number, since it uses as much as digits as its factorization.

21668981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 70890.

The product of its digits is 41472, while the sum is 41.

The square root of 21668981 is about 4654.9952738966. The cubic root of 21668981 is about 278.7914792785.

The spelling of 21668981 in words is "twenty-one million, six hundred sixty-eight thousand, nine hundred eighty-one".