399 (number)

← 398 399 400 →
Cardinalthree hundred ninety-nine
Ordinal399th
(three hundred ninety-ninth)
Factorization3 × 7 × 19
Divisors1, 3, 7, 19, 21, 57, 133, 399
Greek numeralΤϞΘ´
Roman numeralCCCXCIX, cccxcix
Binary1100011112
Ternary1122103
Senary15036
Octal6178
Duodecimal29312
Hexadecimal18F16

399 (three hundred [and] ninety-nine) is the natural number and integer following 398 and preceding 400.

In mathematics

399 is an odd composite number, which is also a square-free number, and a Harshad number. Since 399 is the product of three distinct primes, 3, 7, and 19, it is a sphenic number. 399 is a Leyland number of the second kind, since it is a number of the form of .

Lucas–Carmichael number

A Lucas–Carmichael number is a positive composite square-free number n such that if p is a prime factor of n, then p + 1 divides n + 1.

399 satisfies this equation because 399 is a composite number that is also square-free. When you add 1 to the number (399 + 1 = 400) and the prime factors (3 + 1, 7 + 1, 19 + 1), the product of the prime factors will be the prime factors of 400, meaning that 399 is a Lucas–Carmichael number, and it is also the smallest one.

References

  1. 1 2 3 "Properties of the number 399". Number Empire. Retrieved 2026-10-20.
  2. ↑ Sloane, N. J. A. (ed.). "Sequence A005117 (Squarefree numbers: numbers that are not divisible by a square greater than 1)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  3. ↑ "Number 399 - Harshad Number, Properties & Facts | NumberWorld". numberworld.info. Retrieved 2026-10-20.
  4. ↑ Weisstein, Eric W. "Sphenic Number". mathworld.wolfram.com. Wolfram Research, Inc. Retrieved 2026-10-20.
  5. ↑ Sloane, N. J. A. (ed.). "Sequence A045575 (Nonnegative numbers of the form x^y - y^x, for x,y > 1)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  6. 1 2 Weisstein, Eric W. "Lucas-Carmichael Number". mathworld.wolfram.com. Wolfram Research, Inc. Retrieved 2026-10-20.
  7. 1 2 PrimeFan. "Lucas-Carmichael number". PlanetMath. Retrieved 2026-10-20.
  8. ↑ Numberphile (2015-01-15). Something special about 399 (and 2015) - Numberphile. Retrieved 2026-10-20 – via YouTube.