18661038561721 has 4 divisors (see below), whose sum is σ = 18661073577444. Its totient is φ = 18661003546000.

The previous prime is 18661038561601. The next prime is 18661038561727. The reversal of 18661038561721 is 12716583016681.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 6719832645696 + 11941205916025 = 2592264^2 + 3455605^2 .

It is a cyclic number.

It is not a de Polignac number, because 18661038561721 - 2⁴¹ = 16462015306169 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18661038561727) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 16695910 + ... + 17778511.

It is an arithmetic number, because the mean of its divisors is an integer number (4665268394361).

Almost surely, 2^{18661038561721} is an apocalyptic number.

It is an amenable number.

18661038561721 is a deficient number, since it is larger than the sum of its proper divisors (35015723).

18661038561721 is an equidigital number, since it uses as much as digits as its factorization.

18661038561721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 35015722.

The product of its (nonzero) digits is 2903040, while the sum is 55.

The spelling of 18661038561721 in words is "eighteen trillion, six hundred sixty-one billion, thirty-eight million, five hundred sixty-one thousand, seven hundred twenty-one".