638

638 is a positive integer that follows 637 and precedes 639. It is even and composite, with a prime factorization of 2 × 11 × 29. The sum of its proper divisors is 442, which means 638 is a deficient number (the sum of proper divisors is less than the number itself). Its divisors are 1, 2, 11, 22, 29, 58, and 319.

In numeral systems, 638 can be written as DCXXXVIII in Roman numerals. In binary, it is 1001111110, and in hexadecimal, it is 0x27E.

In historical terms, the year 638 CE falls in the early medieval period and is notable in the context of the Islamic conquests. Jerusalem came under Rashidun Caliphate control during this era, with siege operations led by commanders such as Abu Ubaidah ibn al-Jarrah. The change of control helped shape subsequent religious and political developments in the Levant. The year 638 is also represented in the Islamic calendar as a year in the 17th century AH, reflecting the lunar-based dating used in many historical chronicles of the period.

As a number, 638 has no widely recognized special mathematical constants or widely cited unique properties beyond its factorization and basic representations. It appears in ordinary arithmetic, numeration systems, and, like many integers, in historical timelines when specific years are referenced.