1991771991 has 16 divisors (see below), whose sum is σ = 3219025920. Its totient is φ = 1207134360.

The previous prime is 1991771989. The next prime is 1991772001.

1991771991 is nontrivially palindromic in base 10.

It is not a de Polignac number, because 1991771991 - 2¹ = 1991771989 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991771921) 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 15 ways as a sum of consecutive naturals, for example, 3352855 + ... + 3353448.

It is an arithmetic number, because the mean of its divisors is an integer number (201189120).

Almost surely, 2^1991771991 is an apocalyptic number.

1991771991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

1991771991 is a deficient number, since it is larger than the sum of its proper divisors (1227253929).

1991771991 is a wasteful number, since it uses less digits than its factorization.

1991771991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6706323 (or 6706317 counting only the distinct ones).

The product of its digits is 321489, while the sum is 54.

The square root of 1991771991 is about 44629.2728038448. The cubic root of 1991771991 is about 1258.1909014856.

The spelling of 1991771991 in words is "one billion, nine hundred ninety-one million, seven hundred seventy-one thousand, nine hundred ninety-one".