643521661 has 2 divisors, whose sum is σ = 643521662. Its totient is φ = 643521660.

The previous prime is 643521631. The next prime is 643521677. The reversal of 643521661 is 166125346.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 469545561 + 173976100 = 21669^2 + 13190^2 .

It is a cyclic number.

It is not a de Polignac number, because 643521661 - 2¹⁵ = 643488893 is a prime.

It is equal to p_33474049 and since 643521661 and 33474049 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (643521631) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 321760830 + 321760831.

It is an arithmetic number, because the mean of its divisors is an integer number (321760831).

Almost surely, 2^643521661 is an apocalyptic number.

It is an amenable number.

643521661 is a deficient number, since it is larger than the sum of its proper divisors (1).

643521661 is an equidigital number, since it uses as much as digits as its factorization.

643521661 is an evil number, because the sum of its binary digits is even.

The product of its digits is 25920, while the sum is 34.

The square root of 643521661 is about 25367.7287315991. The cubic root of 643521661 is about 863.3516498883.

The spelling of 643521661 in words is "six hundred forty-three million, five hundred twenty-one thousand, six hundred sixty-one".