6438721921 has 4 divisors (see below), whose sum is σ = 6440067844. Its totient is φ = 6437376000.

The previous prime is 6438721903. The next prime is 6438721931. The reversal of 6438721921 is 1291278346.

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 4684033600 + 1754688321 = 68440^2 + 41889^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-6438721921 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (6438721931) 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, 665760 + ... + 675361.

It is an arithmetic number, because the mean of its divisors is an integer number (1610016961).

Almost surely, 2^6438721921 is an apocalyptic number.

It is an amenable number.

6438721921 is a deficient number, since it is larger than the sum of its proper divisors (1345923).

6438721921 is a wasteful number, since it uses less digits than its factorization.

6438721921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1345922.

The product of its digits is 145152, while the sum is 43.

The square root of 6438721921 is about 80241.6470481508. The cubic root of 6438721921 is about 1860.3724078960.

The spelling of 6438721921 in words is "six billion, four hundred thirty-eight million, seven hundred twenty-one thousand, nine hundred twenty-one".