997991 has 2 divisors, whose sum is σ = 997992. Its totient is φ = 997990.

The previous prime is 997973. The next prime is 998009. The reversal of 997991 is 199799.

It is a balanced prime because it is at equal distance from previous prime (997973) and next prime (998009).

It is an emirp because it is prime and its reverse (199799) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 997991 - 2¹⁰ = 996967 is a prime.

It is a Chen prime.

It is an Ulam number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (997961) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 498995 + 498996.

It is an arithmetic number, because the mean of its divisors is an integer number (498996).

2⁹⁹⁷⁹⁹¹ is an apocalyptic number.

997991 is a deficient number, since it is larger than the sum of its proper divisors (1).

997991 is an equidigital number, since it uses as much as digits as its factorization.

997991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 45927, while the sum is 44.

The square root of 997991 is about 998.9949949825. The cubic root of 997991 is about 99.9329884379.

The spelling of 997991 in words is "nine hundred ninety-seven thousand, nine hundred ninety-one".