195991 has 2 divisors, whose sum is σ = 195992. Its totient is φ = 195990.

The previous prime is 195977. The next prime is 195997. The reversal of 195991 is 199591.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 195991 - 2⁷ = 195863 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (195997) 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, 97995 + 97996.

It is an arithmetic number, because the mean of its divisors is an integer number (97996).

2¹⁹⁵⁹⁹¹ is an apocalyptic number.

195991 is a deficient number, since it is larger than the sum of its proper divisors (1).

195991 is an equidigital number, since it uses as much as digits as its factorization.

195991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3645, while the sum is 34.

The square root of 195991 is about 442.7087078430. The cubic root of 195991 is about 58.0869682222.

Subtracting 195991 from its reverse (199591), we obtain a square (3600 = 60²).

It can be divided in two parts, 19599 and 1, that added together give a square (19600 = 140²).

The spelling of 195991 in words is "one hundred ninety-five thousand, nine hundred ninety-one".