197991991 has 2 divisors, whose sum is σ = 197991992. Its totient is φ = 197991990.

The previous prime is 197991973. The next prime is 197992001. The reversal of 197991991 is 199199791.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-197991991 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (197991931) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 98995995 + 98995996.

It is an arithmetic number, because the mean of its divisors is an integer number (98995996).

Almost surely, 2^197991991 is an apocalyptic number.

197991991 is a deficient number, since it is larger than the sum of its proper divisors (1).

197991991 is an equidigital number, since it uses as much as digits as its factorization.

197991991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 413343, while the sum is 55.

The square root of 197991991 is about 14070.9626891695. The cubic root of 197991991 is about 582.8398095879.

The spelling of 197991991 in words is "one hundred ninety-seven million, nine hundred ninety-one thousand, nine hundred ninety-one".