640638 has 12 divisors (see below), whose sum is σ = 1388088. Its totient is φ = 213540.

The previous prime is 640631. The next prime is 640649. The reversal of 640638 is 836046.

It is an Ulam number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (640631) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 17778 + ... + 17813.

It is an arithmetic number, because the mean of its divisors is an integer number (115674).

2⁶⁴⁰⁶³⁸ is an apocalyptic number.

640638 is an abundant number, since it is smaller than the sum of its proper divisors (747450).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

640638 is a wasteful number, since it uses less digits than its factorization.

640638 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 35599 (or 35596 counting only the distinct ones).

The product of its (nonzero) digits is 3456, while the sum is 27.

The square root of 640638 is about 800.3986506735. The cubic root of 640638 is about 86.2060141188.

The spelling of 640638 in words is "six hundred forty thousand, six hundred thirty-eight".