991638091 has 2 divisors, whose sum is σ = 991638092. Its totient is φ = 991638090.

The previous prime is 991638083. The next prime is 991638097. The reversal of 991638091 is 190836199.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 991638091 - 2³ = 991638083 is a prime.

It is a super-2 number, since 2×991638091² = 1966692207044248562, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (991638097) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495819045 + 495819046.

It is an arithmetic number, because the mean of its divisors is an integer number (495819046).

Almost surely, 2^991638091 is an apocalyptic number.

991638091 is a deficient number, since it is larger than the sum of its proper divisors (1).

991638091 is an equidigital number, since it uses as much as digits as its factorization.

991638091 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 104976, while the sum is 46.

The square root of 991638091 is about 31490.2856608193. The cubic root of 991638091 is about 997.2048916484.

The spelling of 991638091 in words is "nine hundred ninety-one million, six hundred thirty-eight thousand, ninety-one".