644720957 has 4 divisors (see below), whose sum is σ = 644777436. Its totient is φ = 644664480.

The previous prime is 644720933. The next prime is 644720959. The reversal of 644720957 is 759027446.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 759027446 = 2 ⋅379513723.

It can be written as a sum of positive squares in 2 ways, for example, as 620059801 + 24661156 = 24901^2 + 4966^2 .

It is a cyclic number.

It is not a de Polignac number, because 644720957 - 2¹⁴ = 644704573 is a prime.

It is a super-2 number, since 2×644720957² = 831330224789991698, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (644720959) 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, 4418 + ... + 36179.

It is an arithmetic number, because the mean of its divisors is an integer number (161194359).

Almost surely, 2^644720957 is an apocalyptic number.

It is an amenable number.

644720957 is a deficient number, since it is larger than the sum of its proper divisors (56479).

644720957 is a wasteful number, since it uses less digits than its factorization.

644720957 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 56478.

The product of its (nonzero) digits is 423360, while the sum is 44.

The square root of 644720957 is about 25391.3559504017. The cubic root of 644720957 is about 863.8876440867.

The spelling of 644720957 in words is "six hundred forty-four million, seven hundred twenty thousand, nine hundred fifty-seven".