640842781 has 4 divisors (see below), whose sum is σ = 640916064. Its totient is φ = 640769500.

The previous prime is 640842779. The next prime is 640842793. The reversal of 640842781 is 187248046.

640842781 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 640842781 - 2¹ = 640842779 is a prime.

It is a super-2 number, since 2×640842781² = 821358939919627922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640842481) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 21415 + ... + 41716.

It is an arithmetic number, because the mean of its divisors is an integer number (160229016).

Almost surely, 2^640842781 is an apocalyptic number.

It is an amenable number.

640842781 is a deficient number, since it is larger than the sum of its proper divisors (73283).

640842781 is a wasteful number, since it uses less digits than its factorization.

640842781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 73282.

The product of its (nonzero) digits is 86016, while the sum is 40.

The square root of 640842781 is about 25314.8727233617. The cubic root of 640842781 is about 862.1519843876.

The spelling of 640842781 in words is "six hundred forty million, eight hundred forty-two thousand, seven hundred eighty-one".