19561998721 has 4 divisors (see below), whose sum is σ = 19670076244. Its totient is φ = 19453921200.

The previous prime is 19561998689. The next prime is 19561998737. The reversal of 19561998721 is 12789916591.

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 10534148496 + 9027850225 = 102636^2 + 95015^2 .

It is a cyclic number.

It is not a de Polignac number, because 19561998721 - 2⁵ = 19561998689 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (19561998751) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 54038490 + ... + 54038851.

It is an arithmetic number, because the mean of its divisors is an integer number (4917519061).

Almost surely, 2^19561998721 is an apocalyptic number.

It is an amenable number.

19561998721 is a deficient number, since it is larger than the sum of its proper divisors (108077523).

19561998721 is a wasteful number, since it uses less digits than its factorization.

19561998721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 108077522.

The product of its digits is 2449440, while the sum is 58.

The spelling of 19561998721 in words is "nineteen billion, five hundred sixty-one million, nine hundred ninety-eight thousand, seven hundred twenty-one".