644629781 has 2 divisors, whose sum is σ = 644629782. Its totient is φ = 644629780.

The previous prime is 644629771. The next prime is 644629787. The reversal of 644629781 is 187926446.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 383807281 + 260822500 = 19591^2 + 16150^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-644629781 is a prime.

It is a super-2 number, since 2×644629781² = 831095109104215922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (644629787) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 322314890 + 322314891.

It is an arithmetic number, because the mean of its divisors is an integer number (322314891).

Almost surely, 2^644629781 is an apocalyptic number.

It is an amenable number.

644629781 is a deficient number, since it is larger than the sum of its proper divisors (1).

644629781 is an equidigital number, since it uses as much as digits as its factorization.

644629781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 580608, while the sum is 47.

The square root of 644629781 is about 25389.5604727612. The cubic root of 644629781 is about 863.8469186988.

The spelling of 644629781 in words is "six hundred forty-four million, six hundred twenty-nine thousand, seven hundred eighty-one".