9981661678561 has 4 divisors (see below), whose sum is σ = 9981668380612. Its totient is φ = 9981654976512.

The previous prime is 9981661678543. The next prime is 9981661678567. The reversal of 9981661678561 is 1658761661899.

9981661678561 is a nontrivial binomial coefficient, being equal to C(4468034, 2).

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 3327614430625 + 6654047247936 = 1824175^2 + 2579544^2 .

It is a 3-Lehmer number, since φ(9981661678561) divides (9981661678561-1)³.

It is a cyclic number.

It is not a de Polignac number, because 9981661678561 - 2³⁵ = 9947301940193 is a prime.

It is not an unprimeable number, because it can be changed into a prime (9981661678567) 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, 1 + ... + 4468033.

It is an arithmetic number, because the mean of its divisors is an integer number (2495417095153).

It is a Poulet number, since it divides 2^{9981661678560}-1.

Almost surely, 2^{9981661678561} is an apocalyptic number.

9981661678561 is the 4468033-rd triangular number and also the 2234017-th hexagonal number.

9981661678561 is the 1489345-th centered nonagonal number.

It is an amenable number.

9981661678561 is a deficient number, since it is larger than the sum of its proper divisors (6702051).

9981661678561 is a wasteful number, since it uses less digits than its factorization.

9981661678561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6702050.

The product of its digits is 235146240, while the sum is 73.

The spelling of 9981661678561 in words is "nine trillion, nine hundred eighty-one billion, six hundred sixty-one million, six hundred seventy-eight thousand, five hundred sixty-one".