21421

21421 is a five-digit natural number. In mathematics, it is notable for being a semiprime, since it factors into two primes: 31 and 691 (21421 = 31 × 691). It is odd, not a multiple of 3 or 5, and its digits sum to 10. The decimal representation 21421 is not a palindrome, though the sequence of digits includes a symmetry around the center in its first and last positions (2 and 1).

In practical coding and postal contexts, five-digit numbers are commonly used as identifiers such as ZIP codes in the United States. The number 21421 may be assigned to a geographic area by the postal service, though ZIP code boundaries can change over time. Beyond postal use, 21421 can appear as an identifier in catalogs, inventories, or datasets where five-digit codes are employed to distinguish items or locations.

As a year label, 21421 would denote a date far in the future; in fiction and speculative contexts it might be used to situate events centuries ahead. In number theory, 21421 serves as an example of a semiprime with a straightforward prime factorization, illustrating how a composite number can be expressed as the product of two primes.