645691021 has 2 divisors, whose sum is σ = 645691022. Its totient is φ = 645691020.

The previous prime is 645691019. The next prime is 645691031. The reversal of 645691021 is 120196546.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 644906025 + 784996 = 25395^2 + 886^2 .

It is a cyclic number.

It is not a de Polignac number, because 645691021 - 2¹ = 645691019 is a prime.

Together with 645691019, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (645691031) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 322845510 + 322845511.

It is an arithmetic number, because the mean of its divisors is an integer number (322845511).

Almost surely, 2^645691021 is an apocalyptic number.

It is an amenable number.

645691021 is a deficient number, since it is larger than the sum of its proper divisors (1).

645691021 is an equidigital number, since it uses as much as digits as its factorization.

645691021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 12960, while the sum is 34.

The square root of 645691021 is about 25410.4510192165. The cubic root of 645691021 is about 864.3207029369.

The spelling of 645691021 in words is "six hundred forty-five million, six hundred ninety-one thousand, twenty-one".