21569981 has 2 divisors, whose sum is σ = 21569982. Its totient is φ = 21569980.

The previous prime is 21569971. The next prime is 21569983. The reversal of 21569981 is 18996512.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20340100 + 1229881 = 4510^2 + 1109^2 .

It is a cyclic number.

It is not a de Polignac number, because 21569981 - 2⁶ = 21569917 is a prime.

It is a super-2 number, since 2×21569981² = 930528160680722, which contains 22 as substring.

Together with 21569983, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21569983) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10784990 + 10784991.

It is an arithmetic number, because the mean of its divisors is an integer number (10784991).

Almost surely, 2^21569981 is an apocalyptic number.

It is an amenable number.

21569981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21569981 is an equidigital number, since it uses as much as digits as its factorization.

21569981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 38880, while the sum is 41.

The square root of 21569981 is about 4644.3493623973. The cubic root of 21569981 is about 278.3662555230.

The spelling of 21569981 in words is "twenty-one million, five hundred sixty-nine thousand, nine hundred eighty-one".