681

681 is a natural number that comes after 680 and before 682. In mathematics and numbering, it serves as a ordinary integer with several notable properties.

In mathematics, 681 factors as 3 × 227, with both 3 and 227 being prime, making 681 a square-free semiprime. It has four positive divisors: 1, 3, 227, and 681. The sum of all divisors is 912, so the sum of proper divisors is 231, which makes 681 a deficient number. In Roman numerals it is DCLXXXI. In binary it is 1010101001. In hexadecimal it is 2A9, and in octal it is 1251.

In the United States, 681 is an overlay area code for Northern Virginia, issued to cover portions of the same region as area code 703. The overlay was introduced in 2009 and, as a result, many callers in the area use ten-digit dialing for local calls.

Beyond mathematics and telephone designation, 681 also appears in various identifiers, model numbers, and catalog references. It does not correspond to a widely recognized mathematical constant or universal standard, but it is used in everyday numbering and coding contexts as part of the broader system of integers.