991811396521 has 6 divisors (see below), whose sum is σ = 1090172857466. Its totient is φ = 901646724000.

The previous prime is 991811396509. The next prime is 991811396531. The reversal of 991811396521 is 125693118199.

It can be written as a sum of positive squares in only one way, i.e., 735786528400 + 256024868121 = 857780^2 + 505989^2 .

It is not a de Polignac number, because 991811396521 - 2¹¹ = 991811394473 is a prime.

It is not an unprimeable number, because it can be changed into a prime (991811396531) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 4098394080 + ... + 4098394321.

Almost surely, 2^991811396521 is an apocalyptic number.

It is an amenable number.

991811396521 is a deficient number, since it is larger than the sum of its proper divisors (98361460945).

991811396521 is a wasteful number, since it uses less digits than its factorization.

991811396521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 8196788423 (or 8196788412 counting only the distinct ones).

The product of its digits is 1049760, while the sum is 55.

The spelling of 991811396521 in words is "nine hundred ninety-one billion, eight hundred eleven million, three hundred ninety-six thousand, five hundred twenty-one".