991020209 has 2 divisors, whose sum is σ = 991020210. Its totient is φ = 991020208.

The previous prime is 991020203. The next prime is 991020211. The reversal of 991020209 is 902020199.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 698439184 + 292581025 = 26428^2 + 17105^2 .

It is an emirp because it is prime and its reverse (902020199) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 991020209 - 2⁴ = 991020193 is a prime.

It is a Sophie Germain prime.

Together with 991020211, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (991020203) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495510104 + 495510105.

It is an arithmetic number, because the mean of its divisors is an integer number (495510105).

Almost surely, 2^991020209 is an apocalyptic number.

It is an amenable number.

991020209 is a deficient number, since it is larger than the sum of its proper divisors (1).

991020209 is an equidigital number, since it uses as much as digits as its factorization.

991020209 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2916, while the sum is 32.

The square root of 991020209 is about 31480.4734557789. The cubic root of 991020209 is about 996.9977317391.

The spelling of 991020209 in words is "nine hundred ninety-one million, twenty thousand, two hundred nine".