6561991656 has 96 divisors (see below), whose sum is σ = 20035545600. Its totient is φ = 1988474400.

The previous prime is 6561991603. The next prime is 6561991661.

6561991656 is a `hidden beast` number, since 6 + 5 + 619 + 9 + 16 + 5 + 6 = 666.

6561991656 is nontrivially palindromic in base 10.

It is a Harshad number since it is a multiple of its sum of digits (54).

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 99400 + ... + 151671.

It is an arithmetic number, because the mean of its divisors is an integer number (208703600).

Almost surely, 2^6561991656 is an apocalyptic number.

6561991656 is a gapful number since it is divisible by the number (66) formed by its first and last digit.

It is an amenable number.

6561991656 is an abundant number, since it is smaller than the sum of its proper divisors (13473553944).

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

6561991656 is a wasteful number, since it uses less digits than its factorization.

6561991656 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 251108 (or 251087 counting only the distinct ones).

The product of its digits is 2624400, while the sum is 54.

The square root of 6561991656 is about 81006.1211020501. The cubic root of 6561991656 is about 1872.1697536726.

The spelling of 6561991656 in words is "six billion, five hundred sixty-one million, nine hundred ninety-one thousand, six hundred fifty-six".