644400661997 has 4 divisors (see below), whose sum is σ = 660117751356. Its totient is φ = 628683572640.

The previous prime is 644400661991. The next prime is 644400662027. The reversal of 644400661997 is 799166004446.

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 45230230276 + 599170431721 = 212674^2 + 774061^2 .

It is a cyclic number.

It is not a de Polignac number, because 644400661997 - 2¹⁰ = 644400660973 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 (644400661991) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 7858544618 + ... + 7858544699.

It is an arithmetic number, because the mean of its divisors is an integer number (165029437839).

Almost surely, 2^644400661997 is an apocalyptic number.

It is an amenable number.

644400661997 is a deficient number, since it is larger than the sum of its proper divisors (15717089359).

644400661997 is a wasteful number, since it uses less digits than its factorization.

644400661997 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 15717089358.

The product of its (nonzero) digits is 7838208, while the sum is 56.

The spelling of 644400661997 in words is "six hundred forty-four billion, four hundred million, six hundred sixty-one thousand, nine hundred ninety-seven".