21005681 has 2 divisors, whose sum is σ = 21005682. Its totient is φ = 21005680.

The previous prime is 21005669. The next prime is 21005689. The reversal of 21005681 is 18650012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12250000 + 8755681 = 3500^2 + 2959^2 .

It is a cyclic number.

It is not a de Polignac number, because 21005681 - 2¹⁸ = 20743537 is a prime.

It is a super-2 number, since 2×21005681² = 882477268547522, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21005689) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10502840 + 10502841.

It is an arithmetic number, because the mean of its divisors is an integer number (10502841).

Almost surely, 2^21005681 is an apocalyptic number.

It is an amenable number.

21005681 is a deficient number, since it is larger than the sum of its proper divisors (1).

21005681 is an equidigital number, since it uses as much as digits as its factorization.

21005681 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 480, while the sum is 23.

The square root of 21005681 is about 4583.1955009578. The cubic root of 21005681 is about 275.9172938843.

Adding to 21005681 its reverse (18650012), we get a palindrome (39655693).

The spelling of 21005681 in words is "twenty-one million, five thousand, six hundred eighty-one".