644707661 has 4 divisors (see below), whose sum is σ = 694300572. Its totient is φ = 595114752.

The previous prime is 644707649. The next prime is 644707669. The reversal of 644707661 is 166707446.

It is a happy number.

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 177715561 + 466992100 = 13331^2 + 21610^2 .

It is a cyclic number.

It is not a de Polignac number, because 644707661 - 2¹⁰ = 644706637 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 (644707669) 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, 24796436 + ... + 24796461.

It is an arithmetic number, because the mean of its divisors is an integer number (173575143).

Almost surely, 2^644707661 is an apocalyptic number.

It is an amenable number.

644707661 is a deficient number, since it is larger than the sum of its proper divisors (49592911).

644707661 is a wasteful number, since it uses less digits than its factorization.

644707661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 49592910.

The product of its (nonzero) digits is 169344, while the sum is 41.

The square root of 644707661 is about 25391.0941276661. The cubic root of 644707661 is about 863.8817054301.

The spelling of 644707661 in words is "six hundred forty-four million, seven hundred seven thousand, six hundred sixty-one".