21779981 has 2 divisors, whose sum is σ = 21779982. Its totient is φ = 21779980.

The previous prime is 21779971. The next prime is 21779987. The reversal of 21779981 is 18997712.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18748900 + 3031081 = 4330^2 + 1741^2 .

It is a cyclic number.

It is not a de Polignac number, because 21779981 - 2⁶ = 21779917 is a prime.

It is a super-2 number, since 2×21779981² = 948735144720722, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21779987) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10889990 + 10889991.

It is an arithmetic number, because the mean of its divisors is an integer number (10889991).

Almost surely, 2^21779981 is an apocalyptic number.

It is an amenable number.

21779981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21779981 is an equidigital number, since it uses as much as digits as its factorization.

21779981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 63504, while the sum is 44.

The square root of 21779981 is about 4666.9027202203. The cubic root of 21779981 is about 279.2667079585.

The spelling of 21779981 in words is "twenty-one million, seven hundred seventy-nine thousand, nine hundred eighty-one".