857

857 is a natural number that follows 856 and precedes 858. In mathematics, 857 is a prime number. It is odd and not divisible by any integer other than 1 and itself. The sum of its digits is 20, giving a digital root of 2. In base representations, 857 is 0x359 in hexadecimal and 1101011001 in binary. In Roman numerals, it is written as DCCCLVII.

In the North American Numbering Plan, 857 is an area code serving the Greater Boston area. It was introduced in 2001 as an overlay for the 617 area code, and is used for phone numbers in and around Boston.

As a year, 857 CE was a year in the 9th century. Historical records from that era are regionally varied, reflecting the broader political and cultural changes of the late early medieval period in Europe and the wider Afro-Easian world.

857 is also used in various cataloging and designation schemes as a numeric identifier, separate from its mathematical properties.