• 638 can be written using four 4's:

See also 113.

---

638 has 8 divisors (see below), whose sum is σ = 1080. Its totient is φ = 280.

The previous prime is 631. The next prime is 641. The reversal of 638 is 836.

It is a happy number.

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

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is a plaindrome in base 9, base 14 and base 16.

It is a nialpdrome in base 11.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (631) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 8 + ... + 36.

It is an arithmetic number, because the mean of its divisors is an integer number (135).

638 is the 14-th centered heptagonal number.

638 is a deficient number, since it is larger than the sum of its proper divisors (442).

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

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

The sum of its prime factors is 42.

The product of its digits is 144, while the sum is 17.

The square root of 638 is about 25.2586618806. The cubic root of 638 is about 8.6087525818.

Subtracting from 638 its product of digits (144), we obtain a palindrome (494).

It can be divided in two parts, 6 and 38, that added together give a palindrome (44).

The spelling of 638 in words is "six hundred thirty-eight", and thus it is an aban number and an oban number.