6645537721 has 4 divisors (see below), whose sum is σ = 6645711744. Its totient is φ = 6645363700.

The previous prime is 6645537719. The next prime is 6645537751. The reversal of 6645537721 is 1277355466.

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, and also an emirpimes, since its reverse is a distinct semiprime: 1277355466 = 2 ⋅638677733.

It is a cyclic number.

It is not a de Polignac number, because 6645537721 - 2¹ = 6645537719 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (6645537751) 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, 2125 + ... + 115306.

It is an arithmetic number, because the mean of its divisors is an integer number (1661427936).

Almost surely, 2^6645537721 is an apocalyptic number.

It is an amenable number.

6645537721 is a deficient number, since it is larger than the sum of its proper divisors (174023).

6645537721 is a wasteful number, since it uses less digits than its factorization.

6645537721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 174022.

The product of its digits is 1058400, while the sum is 46.

The square root of 6645537721 is about 81520.1675722026. The cubic root of 6645537721 is about 1880.0816435986.

The spelling of 6645537721 in words is "six billion, six hundred forty-five million, five hundred thirty-seven thousand, seven hundred twenty-one".