991991891977 has 2 divisors, whose sum is σ = 991991891978. Its totient is φ = 991991891976.

The previous prime is 991991891953. The next prime is 991991891999. The reversal of 991991891977 is 779198199199.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 882944001801 + 109047890176 = 939651^2 + 330224^2 .

It is a cyclic number.

It is not a de Polignac number, because 991991891977 - 2¹¹ = 991991889929 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 991991891897 and 991991891906.

It is not a weakly prime, because it can be changed into another prime (991991891477) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495995945988 + 495995945989.

It is an arithmetic number, because the mean of its divisors is an integer number (495995945989).

Almost surely, 2^991991891977 is an apocalyptic number.

It is an amenable number.

991991891977 is a deficient number, since it is larger than the sum of its proper divisors (1).

991991891977 is an equidigital number, since it uses as much as digits as its factorization.

991991891977 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 208324872, while the sum is 79.

The spelling of 991991891977 in words is "nine hundred ninety-one billion, nine hundred ninety-one million, eight hundred ninety-one thousand, nine hundred seventy-seven".