991557221 has 2 divisors, whose sum is σ = 991557222. Its totient is φ = 991557220.

The previous prime is 991557211. The next prime is 991557223. The reversal of 991557221 is 122755199.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 991557121 + 100 = 31489^2 + 10^2 .

It is a cyclic number.

It is not a de Polignac number, because 991557221 - 2⁶ = 991557157 is a prime.

It is a Sophie Germain prime.

Together with 991557223, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991557223) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495778610 + 495778611.

It is an arithmetic number, because the mean of its divisors is an integer number (495778611).

Almost surely, 2^991557221 is an apocalyptic number.

It is an amenable number.

991557221 is a deficient number, since it is larger than the sum of its proper divisors (1).

991557221 is an equidigital number, since it uses as much as digits as its factorization.

991557221 is an evil number, because the sum of its binary digits is even.

The product of its digits is 56700, while the sum is 41.

The square root of 991557221 is about 31489.0015878560. The cubic root of 991557221 is about 997.1777829170.

The spelling of 991557221 in words is "nine hundred ninety-one million, five hundred fifty-seven thousand, two hundred twenty-one".