6561621555 has 16 divisors (see below), whose sum is σ = 10646463744. Its totient is φ = 3450241760.

The previous prime is 6561621533. The next prime is 6561621559. The reversal of 6561621555 is 5551261656.

It is not a de Polignac number, because 6561621555 - 2⁶ = 6561621491 is a prime.

It is a Smith number, since the sum of its digits (42) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is not an unprimeable number, because it can be changed into a prime (6561621559) 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, 3079509 + ... + 3081638.

It is an arithmetic number, because the mean of its divisors is an integer number (665403984).

Almost surely, 2^6561621555 is an apocalyptic number.

6561621555 is a deficient number, since it is larger than the sum of its proper divisors (4084842189).

6561621555 is a wasteful number, since it uses less digits than its factorization.

6561621555 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6161226.

The product of its digits is 270000, while the sum is 42.

The square root of 6561621555 is about 81003.8366683949. The cubic root of 6561621555 is about 1872.1345557753.

The spelling of 6561621555 in words is "six billion, five hundred sixty-one million, six hundred twenty-one thousand, five hundred fifty-five".