21107981 has 2 divisors, whose sum is σ = 21107982. Its totient is φ = 21107980.

The previous prime is 21107953. The next prime is 21107987. The reversal of 21107981 is 18970112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17147881 + 3960100 = 4141^2 + 1990^2 .

It is a cyclic number.

It is not a de Polignac number, because 21107981 - 2¹⁰ = 21106957 is a prime.

It is a super-2 number, since 2×21107981² = 891093723792722, which contains 22 as substring.

It is equal to p_1336367 and since 21107981 and 1336367 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21107987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10553990 + 10553991.

It is an arithmetic number, because the mean of its divisors is an integer number (10553991).

Almost surely, 2^21107981 is an apocalyptic number.

It is an amenable number.

21107981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21107981 is an equidigital number, since it uses as much as digits as its factorization.

21107981 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1008, while the sum is 29.

The square root of 21107981 is about 4594.3422815459. The cubic root of 21107981 is about 276.3644846695.

The spelling of 21107981 in words is "twenty-one million, one hundred seven thousand, nine hundred eighty-one".