399991 has 4 divisors (see below), whose sum is σ = 407592. Its totient is φ = 392392.

The previous prime is 399989. The next prime is 400009. The reversal of 399991 is 199993.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199993 = 43 ⋅4651.

It is a cyclic number.

It is not a de Polignac number, because 399991 - 2¹ = 399989 is a prime.

It is a Duffinian number.

399991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (399911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3721 + ... + 3826.

It is an arithmetic number, because the mean of its divisors is an integer number (101898).

2³⁹⁹⁹⁹¹ is an apocalyptic number.

399991 is a deficient number, since it is larger than the sum of its proper divisors (7601).

399991 is an equidigital number, since it uses as much as digits as its factorization.

399991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7600.

The product of its digits is 19683, while the sum is 40.

The square root of 399991 is about 632.4484168689. The cubic root of 399991 is about 73.6800773639.

It can be divided in two parts, 39999 and 1, that added together give a square (40000 = 200²).

The spelling of 399991 in words is "three hundred ninety-nine thousand, nine hundred ninety-one".