640781 has 4 divisors (see below), whose sum is σ = 678492. Its totient is φ = 603072.

The previous prime is 640777. The next prime is 640793. The reversal of 640781 is 187046.

640781 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 an emirpimes, since its reverse is a distinct semiprime: 187046 = 2 ⋅93523.

It can be written as a sum of positive squares in 2 ways, for example, as 292681 + 348100 = 541^2 + 590^2 .

It is a cyclic number.

It is not a de Polignac number, because 640781 - 2² = 640777 is a prime.

It is a super-2 number, since 2×640781² = 821200579922, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 640781.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640741) 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, 18830 + ... + 18863.

It is an arithmetic number, because the mean of its divisors is an integer number (169623).

2⁶⁴⁰⁷⁸¹ is an apocalyptic number.

It is an amenable number.

640781 is a deficient number, since it is larger than the sum of its proper divisors (37711).

640781 is a wasteful number, since it uses less digits than its factorization.

640781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 37710.

The product of its (nonzero) digits is 1344, while the sum is 26.

The square root of 640781 is about 800.4879761745. The cubic root of 640781 is about 86.2124277996.

The spelling of 640781 in words is "six hundred forty thousand, seven hundred eighty-one".