6666721721 has 16 divisors (see below), whose sum is σ = 7443836928. Its totient is φ = 5918682000.

The previous prime is 6666721693. The next prime is 6666721739. The reversal of 6666721721 is 1271276666.

It is a de Polignac number, because none of the positive numbers 2^k-6666721721 is a prime.

It is not an unprimeable number, because it can be changed into a prime (6666721021) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 225446 + ... + 253296.

It is an arithmetic number, because the mean of its divisors is an integer number (465239808).

Almost surely, 2^6666721721 is an apocalyptic number.

It is an amenable number.

6666721721 is a deficient number, since it is larger than the sum of its proper divisors (777115207).

6666721721 is a wasteful number, since it uses less digits than its factorization.

6666721721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 28372.

The product of its digits is 254016, while the sum is 44.

The square root of 6666721721 is about 81649.9952296386. The cubic root of 6666721721 is about 1882.0772385589.

It can be divided in two parts, 66667 and 21721, that added together give a palindrome (88388).

The spelling of 6666721721 in words is "six billion, six hundred sixty-six million, seven hundred twenty-one thousand, seven hundred twenty-one".