• 399 can be written using four 4's:

399 has 8 divisors (see below), whose sum is σ = 640. Its totient is φ = 216.

The previous prime is 397. The next prime is 401. The reversal of 399 is 993.

399 = T_{7} + T_{8} + ... +
T_{13}.

399 is nontrivially palindromic in base 11.

It is a Cunningham number, because it is equal to 20^{2}-1.

It is an interprime number because it is at equal distance from previous prime (397) and next prime (401).

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 399 - 2^{1} = 397 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (21), and also a Moran number because the ratio is a prime number: 19 = 399 / (3 + 9 + 9).

It is a D-number.

It is a Duffinian number.

399 is a modest number, since divided by 99 gives 3 as remainder.

399 is a lucky number.

399 is a nontrivial repdigit in base 11.

It is a plaindrome in base 10, base 11, base 13 and base 16.

It is a nialpdrome in base 7 and base 11.

It is a zygodrome in base 2 and base 11.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (397) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 12 + ... + 30.

It is an arithmetic number, because the mean of its divisors is an integer number (80).

399 is a deficient number, since it is larger than the sum of its proper divisors (241).

399 is a wasteful number, since it uses less digits than its factorization.

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

The sum of its prime factors is 29.

The product of its digits is 243, while the sum is 21.

The square root of 399 is about 19.9749843554. The cubic root of 399 is about 7.3619178210.

Subtracting from 399 its sum of digits (21), we obtain a triangular number (378 = T_{27}).

It can be divided in two parts, 39 and 9, that multiplied together give a triangular number (351 = T_{26}).

The spelling of 399 in words is "three hundred ninety-nine", and thus it is an aban number and an oban number.

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