391991 has 4 divisors (see below), whose sum is σ = 397584. Its totient is φ = 386400.

The previous prime is 391987. The next prime is 391999. The reversal of 391991 is 199193.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 391991 - 2² = 391987 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 (391999) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2690 + ... + 2831.

It is an arithmetic number, because the mean of its divisors is an integer number (99396).

2³⁹¹⁹⁹¹ is an apocalyptic number.

391991 is a deficient number, since it is larger than the sum of its proper divisors (5593).

391991 is an equidigital number, since it uses as much as digits as its factorization.

391991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5592.

The product of its digits is 2187, while the sum is 32.

The square root of 391991 is about 626.0918462973. The cubic root of 391991 is about 73.1855540984.

The spelling of 391991 in words is "three hundred ninety-one thousand, nine hundred ninety-one".