21805981 has 2 divisors, whose sum is σ = 21805982. Its totient is φ = 21805980.

The previous prime is 21805967. The next prime is 21805991. The reversal of 21805981 is 18950812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21068100 + 737881 = 4590^2 + 859^2 .

It is a cyclic number.

It is not a de Polignac number, because 21805981 - 2¹¹ = 21803933 is a prime.

It is a super-2 number, since 2×21805981² = 951001614744722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21805991) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10902990 + 10902991.

It is an arithmetic number, because the mean of its divisors is an integer number (10902991).

Almost surely, 2^21805981 is an apocalyptic number.

It is an amenable number.

21805981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21805981 is an equidigital number, since it uses as much as digits as its factorization.

21805981 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 5760, while the sum is 34.

The square root of 21805981 is about 4669.6874627752. The cubic root of 21805981 is about 279.3777892776.

The spelling of 21805981 in words is "twenty-one million, eight hundred five thousand, nine hundred eighty-one".