399 (number)
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399 is a natural number that follows 398 and precedes 400. It is an odd number, as it is not divisible by two, and it is a composite number with divisors other than one and itself.
In Roman numerals it is written CCCXCIX. In binary it is 110001111. The sum of its digits (3+9+9) is 21.
Divisors and factors
The prime factorization of 399 is 3 × 7 × 19. It is therefore a sphenic number, meaning it is the product of three distinct prime numbers. Its complete divisors are 1, 3, 7, 19, 21, 57, 133, and 399—eight divisors in total. The sum of its proper divisors (those less than the number itself) is 241, which is less than 399, making it a deficient number.
Relationship to 400
399 can be expressed as 20² − 1, which equals (20 − 1)(20 + 1) = 19 × 21. This form illustrates the difference of squares formula, a² − b².
Other properties
399 is a Harshad number, also known as a Niven number: it is divisible by the sum of its digits, since 399 ÷ 21 = 19. It is also counted among Lucas–Carmichael numbers, which are composite numbers with no squared prime divisor such that for each prime factor p of n, p + 1 divides n + 1. Here 3+1=4, 7+1=8, and 19+1=20 all divide 400.