639

639 is a natural number that follows 638 and precedes 640. It is an odd composite number.

In mathematics, the prime factorization of 639 is 3^2 × 71, so its divisors are 1, 3, 9, 71, 213, and 639. Since it is divisible by 9, it is divisible by 3 as well. The sum of its digits is 18, which is divisible by 9, aligning with its divisibility by 9.

In various base systems, 639 has the representations 0b1001111111 in binary, 0o1177 in octal, and 0x27F in hexadecimal. In Roman numerals, 639 is written as DCXXXIX.

As a year, 639 CE falls in the early medieval period. Historical records from that era relate to the broader context of Byzantine–Arab interactions and the ongoing expansion of Muslim states in the Near East and surrounding regions.

Overall, 639 is characterized by its factorization into 3^2 and 71, its non-prime status, and its representations across different numeral systems, along with its place as a year in the historical timeline.