21008681 has 2 divisors, whose sum is σ = 21008682. Its totient is φ = 21008680.

The previous prime is 21008677. The next prime is 21008749. The reversal of 21008681 is 18680012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17977600 + 3031081 = 4240^2 + 1741^2 .

It is a cyclic number.

It is not a de Polignac number, because 21008681 - 2² = 21008677 is a prime.

It is a super-2 number, since 2×21008681² = 882729354719522, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is equal to p_1330487 and since 21008681 and 1330487 have the same sum of digits, it is a Honaker prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21008681.

It is not a weakly prime, because it can be changed into another prime (21008081) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504340 + 10504341.

It is an arithmetic number, because the mean of its divisors is an integer number (10504341).

Almost surely, 2^21008681 is an apocalyptic number.

It is an amenable number.

21008681 is a deficient number, since it is larger than the sum of its proper divisors (1).

21008681 is an equidigital number, since it uses as much as digits as its factorization.

21008681 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 768, while the sum is 26.

The square root of 21008681 is about 4583.5227718426. The cubic root of 21008681 is about 275.9304286244.

Adding to 21008681 its reverse (18680012), we get a palindrome (39688693).

The spelling of 21008681 in words is "twenty-one million, eight thousand, six hundred eighty-one".