640661 has 8 divisors (see below), whose sum is σ = 770880. Its totient is φ = 520128.

The previous prime is 640649. The next prime is 640663. The reversal of 640661 is 166046.

640661 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 640661 - 2¹⁰ = 639637 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640663) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2276 + ... + 2541.

It is an arithmetic number, because the mean of its divisors is an integer number (96360).

2⁶⁴⁰⁶⁶¹ is an apocalyptic number.

It is an amenable number.

640661 is a deficient number, since it is larger than the sum of its proper divisors (130219).

640661 is a wasteful number, since it uses less digits than its factorization.

640661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4843.

The product of its (nonzero) digits is 864, while the sum is 23.

The square root of 640661 is about 800.4130183849. The cubic root of 640661 is about 86.2070457542.

The spelling of 640661 in words is "six hundred forty thousand, six hundred sixty-one".