21012221981 has 2 divisors, whose sum is σ = 21012221982. Its totient is φ = 21012221980.

The previous prime is 21012221953. The next prime is 21012222017. The reversal of 21012221981 is 18912221012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19511899225 + 1500322756 = 139685^2 + 38734^2 .

It is a cyclic number.

It is not a de Polignac number, because 21012221981 - 2⁶ = 21012221917 is a prime.

It is a super-2 number, since 2×21012221981² (a number of 21 digits) 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 (21012221921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10506110990 + 10506110991.

It is an arithmetic number, because the mean of its divisors is an integer number (10506110991).

Almost surely, 2^21012221981 is an apocalyptic number.

It is an amenable number.

21012221981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21012221981 is an equidigital number, since it uses as much as digits as its factorization.

21012221981 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1152, while the sum is 29.

Adding to 21012221981 its reverse (18912221012), we get a palindrome (39924442993).

The spelling of 21012221981 in words is "twenty-one billion, twelve million, two hundred twenty-one thousand, nine hundred eighty-one".