991131881 has 2 divisors, whose sum is σ = 991131882. Its totient is φ = 991131880.

The previous prime is 991131859. The next prime is 991131899. The reversal of 991131881 is 188131199.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 967210000 + 23921881 = 31100^2 + 4891^2 .

It is a cyclic number.

It is not a de Polignac number, because 991131881 - 2¹⁴ = 991115497 is a prime.

It is a super-2 number, since 2×991131881² = 1964684811069196322, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (991131781) 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, 495565940 + 495565941.

It is an arithmetic number, because the mean of its divisors is an integer number (495565941).

Almost surely, 2^991131881 is an apocalyptic number.

It is an amenable number.

991131881 is a deficient number, since it is larger than the sum of its proper divisors (1).

991131881 is an equidigital number, since it uses as much as digits as its factorization.

991131881 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 15552, while the sum is 41.

The square root of 991131881 is about 31482.2470767256. The cubic root of 991131881 is about 997.0351788560.

The spelling of 991131881 in words is "nine hundred ninety-one million, one hundred thirty-one thousand, eight hundred eighty-one".