19999987561 has 2 divisors, whose sum is σ = 19999987562. Its totient is φ = 19999987560.

The previous prime is 19999987543. The next prime is 19999987697. The reversal of 19999987561 is 16578999991.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15947901225 + 4052086336 = 126285^2 + 63656^2 .

It is an emirp because it is prime and its reverse (16578999991) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 19999987561 - 2²⁵ = 19966433129 is a prime.

It is not a weakly prime, because it can be changed into another prime (19999387561) 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 as a sum of consecutive naturals, namely, 9999993780 + 9999993781.

It is an arithmetic number, because the mean of its divisors is an integer number (9999993781).

Almost surely, 2^19999987561 is an apocalyptic number.

It is an amenable number.

19999987561 is a deficient number, since it is larger than the sum of its proper divisors (1).

19999987561 is an equidigital number, since it uses as much as digits as its factorization.

19999987561 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 99202320, while the sum is 73.

The spelling of 19999987561 in words is "nineteen billion, nine hundred ninety-nine million, nine hundred eighty-seven thousand, five hundred sixty-one".