6521721660 has 36 divisors (see below), whose sum is σ = 19782556248. Its totient is φ = 1739125728.

The previous prime is 6521721659. The next prime is 6521721673. The reversal of 6521721660 is 661271256.

It is a happy number.

6521721660 is a `hidden beast` number, since 6 + 521 + 72 + 1 + 6 + 60 = 666.

It is a tau number, because it is divible by the number of its divisors (36).

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 18115714 + ... + 18116073.

Almost surely, 2^6521721660 is an apocalyptic number.

6521721660 is a gapful number since it is divisible by the number (60) formed by its first and last digit.

It is an amenable number.

6521721660 is an abundant number, since it is smaller than the sum of its proper divisors (13260834588).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

6521721660 is a wasteful number, since it uses less digits than its factorization.

6521721660 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 36231802 (or 36231797 counting only the distinct ones).

The product of its (nonzero) digits is 30240, while the sum is 36.

The square root of 6521721660 is about 80757.1771423444. The cubic root of 6521721660 is about 1868.3321474374.

The spelling of 6521721660 in words is "six billion, five hundred twenty-one million, seven hundred twenty-one thousand, six hundred sixty".