991811581 has 2 divisors, whose sum is σ = 991811582. Its totient is φ = 991811580.

The previous prime is 991811477. The next prime is 991811591. The reversal of 991811581 is 185118199.

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., 856089081 + 135722500 = 29259^2 + 11650^2 .

It is a cyclic number.

It is not a de Polignac number, because 991811581 - 2¹⁵ = 991778813 is a prime.

It is a super-2 number, since 2×991811581² = 1967380424411439122, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991811591) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495905790 + 495905791.

It is an arithmetic number, because the mean of its divisors is an integer number (495905791).

Almost surely, 2^991811581 is an apocalyptic number.

It is an amenable number.

991811581 is a deficient number, since it is larger than the sum of its proper divisors (1).

991811581 is an equidigital number, since it uses as much as digits as its factorization.

991811581 is an evil number, because the sum of its binary digits is even.

The product of its digits is 25920, while the sum is 43.

The square root of 991811581 is about 31493.0401993837. The cubic root of 991811581 is about 997.2630429000.

The spelling of 991811581 in words is "nine hundred ninety-one million, eight hundred eleven thousand, five hundred eighty-one".