638901821 has 4 divisors (see below), whose sum is σ = 643932672. Its totient is φ = 633870972.

The previous prime is 638901817. The next prime is 638901839. The reversal of 638901821 is 128109836.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 638901821 - 2² = 638901817 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (638901721) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2515235 + ... + 2515488.

It is an arithmetic number, because the mean of its divisors is an integer number (160983168).

Almost surely, 2^638901821 is an apocalyptic number.

It is an amenable number.

638901821 is a deficient number, since it is larger than the sum of its proper divisors (5030851).

638901821 is a wasteful number, since it uses less digits than its factorization.

638901821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5030850.

The product of its (nonzero) digits is 20736, while the sum is 38.

The square root of 638901821 is about 25276.5072943237. The cubic root of 638901821 is about 861.2806865386.

The spelling of 638901821 in words is "six hundred thirty-eight million, nine hundred one thousand, eight hundred twenty-one".