638821 has 4 divisors (see below), whose sum is σ = 654444. Its totient is φ = 623200.

The previous prime is 638819. The next prime is 638839. The reversal of 638821 is 128836.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 98596 + 540225 = 314^2 + 735^2 .

It is not a de Polignac number, because 638821 - 2¹ = 638819 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is a self number, because there is not a number n which added to its sum of digits gives 638821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (638801) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 7750 + ... + 7831.

It is an arithmetic number, because the mean of its divisors is an integer number (163611).

2⁶³⁸⁸²¹ is an apocalyptic number.

It is an amenable number.

638821 is a deficient number, since it is larger than the sum of its proper divisors (15623).

638821 is a wasteful number, since it uses less digits than its factorization.

638821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 15622.

The product of its digits is 2304, while the sum is 28.

The square root of 638821 is about 799.2627853216. The cubic root of 638821 is about 86.1244367708.

The spelling of 638821 in words is "six hundred thirty-eight thousand, eight hundred twenty-one".