644406563261561 has 4 divisors (see below), whose sum is σ = 644406625991412. Its totient is φ = 644406500531712.

The previous prime is 644406563261497. The next prime is 644406563261647. The reversal of 644406563261561 is 165162365604446.

It is a happy number.

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.

It can be written as a sum of positive squares in 2 ways, for example, as 304999818491536 + 339406744770025 = 17464244^2 + 18422995^2 .

It is a cyclic number.

It is not a de Polignac number, because 644406563261561 - 2⁶ = 644406563261497 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 644406563261561.

It is not an unprimeable number, because it can be changed into a prime (644406563269561) 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, 11949800 + ... + 37836633.

It is an arithmetic number, because the mean of its divisors is an integer number (161101656497853).

Almost surely, 2^{644406563261561} is an apocalyptic number.

It is an amenable number.

644406563261561 is a deficient number, since it is larger than the sum of its proper divisors (62729851).

644406563261561 is a wasteful number, since it uses less digits than its factorization.

644406563261561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 62729850.

The product of its (nonzero) digits is 74649600, while the sum is 59.

The spelling of 644406563261561 in words is "six hundred forty-four trillion, four hundred six billion, five hundred sixty-three million, two hundred sixty-one thousand, five hundred sixty-one".