638129721 has 4 divisors (see below), whose sum is σ = 850839632. Its totient is φ = 425419812.

The previous prime is 638129711. The next prime is 638129743. The reversal of 638129721 is 127921836.

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 not a de Polignac number, because 638129721 - 2¹⁴ = 638113337 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 638129721.

It is not an unprimeable number, because it can be changed into a prime (638129711) 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, 106354951 + ... + 106354956.

It is an arithmetic number, because the mean of its divisors is an integer number (212709908).

Almost surely, 2^638129721 is an apocalyptic number.

It is an amenable number.

638129721 is a deficient number, since it is larger than the sum of its proper divisors (212709911).

638129721 is a wasteful number, since it uses less digits than its factorization.

638129721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 212709910.

The product of its digits is 36288, while the sum is 39.

The square root of 638129721 is about 25261.2296019018. The cubic root of 638129721 is about 860.9335998899.

The spelling of 638129721 in words is "six hundred thirty-eight million, one hundred twenty-nine thousand, seven hundred twenty-one".