1991238581 has 2 divisors, whose sum is σ = 1991238582. Its totient is φ = 1991238580.

The previous prime is 1991238527. The next prime is 1991238607. The reversal of 1991238581 is 1858321991.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1817657956 + 173580625 = 42634^2 + 13175^2 .

It is an emirp because it is prime and its reverse (1858321991) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 1991238581 - 2⁶ = 1991238517 is a prime.

It is a super-2 number, since 2×1991238581² = 7930062172925787122, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1991238511) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 995619290 + 995619291.

It is an arithmetic number, because the mean of its divisors is an integer number (995619291).

Almost surely, 2^1991238581 is an apocalyptic number.

It is an amenable number.

1991238581 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991238581 is an equidigital number, since it uses as much as digits as its factorization.

1991238581 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 155520, while the sum is 47.

The square root of 1991238581 is about 44623.2963932518. The cubic root of 1991238581 is about 1258.0785741154.

The spelling of 1991238581 in words is "one billion, nine hundred ninety-one million, two hundred thirty-eight thousand, five hundred eighty-one".