21012991 has 2 divisors, whose sum is σ = 21012992. Its totient is φ = 21012990.

The previous prime is 21012913. The next prime is 21012997. The reversal of 21012991 is 19921012.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21012991 - 2¹¹ = 21010943 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21012997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10506495 + 10506496.

It is an arithmetic number, because the mean of its divisors is an integer number (10506496).

Almost surely, 2^21012991 is an apocalyptic number.

21012991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21012991 is an equidigital number, since it uses as much as digits as its factorization.

21012991 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 324, while the sum is 25.

The square root of 21012991 is about 4583.9929101167. The cubic root of 21012991 is about 275.9492966791.

The spelling of 21012991 in words is "twenty-one million, twelve thousand, nine hundred ninety-one".