645443021 has 4 divisors (see below), whose sum is σ = 645626844. Its totient is φ = 645259200.

The previous prime is 645443017. The next prime is 645443023. The reversal of 645443021 is 120344546.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 258888100 + 386554921 = 16090^2 + 19661^2 .

It is a cyclic number.

It is not a de Polignac number, because 645443021 - 2² = 645443017 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645443023) 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 in 3 ways as a sum of consecutive naturals, for example, 86540 + ... + 93701.

It is an arithmetic number, because the mean of its divisors is an integer number (161406711).

Almost surely, 2^645443021 is an apocalyptic number.

It is an amenable number.

645443021 is a deficient number, since it is larger than the sum of its proper divisors (183823).

645443021 is a wasteful number, since it uses less digits than its factorization.

645443021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 183822.

The product of its (nonzero) digits is 11520, while the sum is 29.

The square root of 645443021 is about 25405.5706686545. The cubic root of 645443021 is about 864.2100313328.

Adding to 645443021 its reverse (120344546), we get a palindrome (765787567).

The spelling of 645443021 in words is "six hundred forty-five million, four hundred forty-three thousand, twenty-one".