640543681 has 2 divisors, whose sum is σ = 640543682. Its totient is φ = 640543680.

The previous prime is 640543649. The next prime is 640543691. The reversal of 640543681 is 186345046.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 629759025 + 10784656 = 25095^2 + 3284^2 .

It is a cyclic number.

It is not a de Polignac number, because 640543681 - 2⁵ = 640543649 is a prime.

It is a super-2 number, since 2×640543681² = 820592414538059522, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 640543681.

It is not a weakly prime, because it can be changed into another prime (640543621) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 320271840 + 320271841.

It is an arithmetic number, because the mean of its divisors is an integer number (320271841).

Almost surely, 2^640543681 is an apocalyptic number.

It is an amenable number.

640543681 is a deficient number, since it is larger than the sum of its proper divisors (1).

640543681 is an equidigital number, since it uses as much as digits as its factorization.

640543681 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 69120, while the sum is 37.

The square root of 640543681 is about 25308.9644395025. The cubic root of 640543681 is about 862.0178330299.

It can be divided in two parts, 6405 and 43681, that added together give a triangular number (50086 = T₃₁₆).

The spelling of 640543681 in words is "six hundred forty million, five hundred forty-three thousand, six hundred eighty-one".