16566661561 has 4 divisors (see below), whose sum is σ = 16572708324. Its totient is φ = 16560614800.

The previous prime is 16566661549. The next prime is 16566661603. The reversal of 16566661561 is 16516666561.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 16516666561 = 97 ⋅170274913.

It can be written as a sum of positive squares in 2 ways, for example, as 4338725161 + 12227936400 = 65869^2 + 110580^2 .

It is a cyclic number.

It is not a de Polignac number, because 16566661561 - 2²¹ = 16564564409 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (16566661511) 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, 3019270 + ... + 3024751.

It is an arithmetic number, because the mean of its divisors is an integer number (4143177081).

Almost surely, 2^16566661561 is an apocalyptic number.

It is an amenable number.

16566661561 is a deficient number, since it is larger than the sum of its proper divisors (6046763).

16566661561 is an equidigital number, since it uses as much as digits as its factorization.

16566661561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6046762.

The product of its digits is 1166400, while the sum is 49.

Subtracting from 16566661561 its reverse (16516666561), we obtain a triangular number (49995000 = T₉₉₉₉).

The spelling of 16566661561 in words is "sixteen billion, five hundred sixty-six million, six hundred sixty-one thousand, five hundred sixty-one".