638991 has 6 divisors (see below), whose sum is σ = 923000. Its totient is φ = 425988.

The previous prime is 638977. The next prime is 638993. The reversal of 638991 is 199836.

It is not a de Polignac number, because 638991 - 2⁵ = 638959 is a prime.

It is a Duffinian number.

638991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (638993) 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, 35491 + ... + 35508.

2⁶³⁸⁹⁹¹ is an apocalyptic number.

638991 is a deficient number, since it is larger than the sum of its proper divisors (284009).

638991 is a wasteful number, since it uses less digits than its factorization.

638991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 71005 (or 71002 counting only the distinct ones).

The product of its digits is 11664, while the sum is 36.

The square root of 638991 is about 799.3691262489. The cubic root of 638991 is about 86.1320757681.

The spelling of 638991 in words is "six hundred thirty-eight thousand, nine hundred ninety-one".