644020657 has 4 divisors (see below), whose sum is σ = 736023616. Its totient is φ = 552017700.

The previous prime is 644020651. The next prime is 644020679. The reversal of 644020657 is 756020446.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 756020446 = 2 ⋅378010223.

It is a cyclic number.

It is not a de Polignac number, because 644020657 - 2⁷ = 644020529 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (644020651) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 46001469 + ... + 46001482.

It is an arithmetic number, because the mean of its divisors is an integer number (184005904).

Almost surely, 2^644020657 is an apocalyptic number.

It is an amenable number.

644020657 is a deficient number, since it is larger than the sum of its proper divisors (92002959).

644020657 is an equidigital number, since it uses as much as digits as its factorization.

644020657 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 92002958.

The product of its (nonzero) digits is 40320, while the sum is 34.

The square root of 644020657 is about 25377.5620775519. The cubic root of 644020657 is about 863.5747440186.

The spelling of 644020657 in words is "six hundred forty-four million, twenty thousand, six hundred fifty-seven".