640409681 has 2 divisors, whose sum is σ = 640409682. Its totient is φ = 640409680.

The previous prime is 640409641. The next prime is 640409699. The reversal of 640409681 is 186904046.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 442050625 + 198359056 = 21025^2 + 14084^2 .

It is a cyclic number.

It is not a de Polignac number, because 640409681 - 2¹⁴ = 640393297 is a prime.

It is a super-2 number, since 2×640409681² = 820249119037043522, which contains 22 as substring.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (640409641) 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, 320204840 + 320204841.

It is an arithmetic number, because the mean of its divisors is an integer number (320204841).

Almost surely, 2^640409681 is an apocalyptic number.

It is an amenable number.

640409681 is a deficient number, since it is larger than the sum of its proper divisors (1).

640409681 is an equidigital number, since it uses as much as digits as its factorization.

640409681 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 41472, while the sum is 38.

The square root of 640409681 is about 25306.3170176934. The cubic root of 640409681 is about 861.9577182405.

The spelling of 640409681 in words is "six hundred forty million, four hundred nine thousand, six hundred eighty-one".