21707981 has 2 divisors, whose sum is σ = 21707982. Its totient is φ = 21707980.

The previous prime is 21707977. The next prime is 21707999. The reversal of 21707981 is 18970712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16532356 + 5175625 = 4066^2 + 2275^2 .

It is a cyclic number.

It is not a de Polignac number, because 21707981 - 2² = 21707977 is a prime.

It is a super-2 number, since 2×21707981² = 942472878192722, which contains 22 as substring.

It is a Sophie Germain 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 (21707941) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10853990 + 10853991.

It is an arithmetic number, because the mean of its divisors is an integer number (10853991).

Almost surely, 2^21707981 is an apocalyptic number.

It is an amenable number.

21707981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21707981 is an equidigital number, since it uses as much as digits as its factorization.

21707981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 7056, while the sum is 35.

The square root of 21707981 is about 4659.1824390122. The cubic root of 21707981 is about 278.9586360572.

The spelling of 21707981 in words is "twenty-one million, seven hundred seven thousand, nine hundred eighty-one".