644747821 has 2 divisors, whose sum is σ = 644747822. Its totient is φ = 644747820.

The previous prime is 644747819. The next prime is 644747839. The reversal of 644747821 is 128747446.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 502208100 + 142539721 = 22410^2 + 11939^2 .

It is a cyclic number.

It is not a de Polignac number, because 644747821 - 2¹ = 644747819 is a prime.

Together with 644747819, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (644747801) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 322373910 + 322373911.

It is an arithmetic number, because the mean of its divisors is an integer number (322373911).

Almost surely, 2^644747821 is an apocalyptic number.

It is an amenable number.

644747821 is a deficient number, since it is larger than the sum of its proper divisors (1).

644747821 is an equidigital number, since it uses as much as digits as its factorization.

644747821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 301056, while the sum is 43.

The square root of 644747821 is about 25391.8849438162. The cubic root of 644747821 is about 863.8996426395.

The spelling of 644747821 in words is "six hundred forty-four million, seven hundred forty-seven thousand, eight hundred twenty-one".