991091009 has 2 divisors, whose sum is σ = 991091010. Its totient is φ = 991091008.

The previous prime is 991090967. The next prime is 991091011. The reversal of 991091009 is 900190199.

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., 702091009 + 289000000 = 26497^2 + 17000^2 .

It is a cyclic number.

It is not a de Polignac number, because 991091009 - 2¹² = 991086913 is a prime.

It is a super-2 number, since 2×991091009² = 1964522776241276162, which contains 22 as substring.

Together with 991091011, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (991091039) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495545504 + 495545505.

It is an arithmetic number, because the mean of its divisors is an integer number (495545505).

Almost surely, 2^991091009 is an apocalyptic number.

It is an amenable number.

991091009 is a deficient number, since it is larger than the sum of its proper divisors (1).

991091009 is an equidigital number, since it uses as much as digits as its factorization.

991091009 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 6561, while the sum is 38.

The square root of 991091009 is about 31481.5979422900. The cubic root of 991091009 is about 997.0214735215.

The spelling of 991091009 in words is "nine hundred ninety-one million, ninety-one thousand, nine".