391951991 has 4 divisors (see below), whose sum is σ = 392810112. Its totient is φ = 391093872.

The previous prime is 391951957. The next prime is 391951997. The reversal of 391951991 is 199159193.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199159193 = 5813 ⋅34261.

It is a cyclic number.

It is not a de Polignac number, because 391951991 - 2¹⁰ = 391950967 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (391951997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 428375 + ... + 429288.

It is an arithmetic number, because the mean of its divisors is an integer number (98202528).

Almost surely, 2^391951991 is an apocalyptic number.

391951991 is a deficient number, since it is larger than the sum of its proper divisors (858121).

391951991 is an equidigital number, since it uses as much as digits as its factorization.

391951991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 858120.

The product of its digits is 98415, while the sum is 47.

The square root of 391951991 is about 19797.7774257617. Note that the first 3 decimals coincide. The cubic root of 391951991 is about 731.8312633005.

The spelling of 391951991 in words is "three hundred ninety-one million, nine hundred fifty-one thousand, nine hundred ninety-one".