637

637 is a natural number that comes after 636 and before 638. In mathematics, it is an odd composite number with prime factorization 7^2 × 13. Its positive divisors are 1, 7, 13, 49, 91, and 637. The sum of its proper divisors is 161, which is less than 637, so 637 is a deficient number. In Roman numerals, 637 is written as DCXXXVII, and in binary it is 1001111101.

As a calendar year, 637 refers to a year in the 7th century CE. During the early stages of the Islamic expansion, Arab Muslim forces captured the Sassanian capital of Ctesiphon in 637, marking a significant development in the Persian theater of the conquests that followed the Battle of Qadisiyya in the prior year. The events of that year are situated within the broader context of the Rashidun Caliphate’s western and eastern campaigns.

Other uses of the number 637 are less common in general reference materials, but it also appears in various numeric and classificatory systems as a distinct value. In everyday notation, 637 is simply the three-digit decimal representation of one of the many integers used in counting, labeling, and indexing.