643817561 has 2 divisors, whose sum is σ = 643817562. Its totient is φ = 643817560.

The previous prime is 643817521. The next prime is 643817569. The reversal of 643817561 is 165718346.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 599515225 + 44302336 = 24485^2 + 6656^2 .

It is a cyclic number.

It is not a de Polignac number, because 643817561 - 2⁶ = 643817497 is a prime.

It is not a weakly prime, because it can be changed into another prime (643817569) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 321908780 + 321908781.

It is an arithmetic number, because the mean of its divisors is an integer number (321908781).

Almost surely, 2^643817561 is an apocalyptic number.

It is an amenable number.

643817561 is a deficient number, since it is larger than the sum of its proper divisors (1).

643817561 is an equidigital number, since it uses as much as digits as its factorization.

643817561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 120960, while the sum is 41.

The square root of 643817561 is about 25373.5602744274. The cubic root of 643817561 is about 863.4839565497.

The spelling of 643817561 in words is "six hundred forty-three million, eight hundred seventeen thousand, five hundred sixty-one".