21999

21999 is a natural number that comes after 21998 and before 22000. It is an odd, composite integer and does not belong to a special named category in common number series, but it has several simple mathematical properties worth noting.

In terms of prime factorization, 21999 = 3 × 7333, where 7333 is a prime number. This makes 21999 a semiprime, a product of two distinct primes. Consequently, its positive divisors are exactly 1, 3, 7333, and 21999. The sum of its digits is 2 + 1 + 9 + 9 + 9 = 30, which shows that the number is divisible by 3 but not by 9. The number is square-free, as no prime factor is repeated.

From a numerical perspective, 21999 is congruent to 3 modulo 6, and it ends in the digit 9, which influences certain modular properties but does not yield additional simple divisibility by small primes beyond 3. As a five-digit decimal, it is not a repunit or a simple repeating-digit number.

In usage, 21999 does not have a widely recognized historical year, geographic designation, or standard technical meaning. It may appear in fiction as a distant future year or as a catalog, part, or model number in various contexts, but there is no canonical or universally accepted designation tied to the sequence.