661

661 is a natural number that follows 660 and precedes 662. In mathematics, it is prime and odd. Its representations include binary 1010010101, hexadecimal 0x295, and Roman numeral DCLXI.

As a prime number, 661 has no divisors other than 1 and itself. It is not divisible by any prime less than or equal to the square root of 661 (2, 3, 5, 7, 11, 13, 17, 19, and 23).

Historically, the year 661 CE marks the end of the Rashidun Caliphate and the rise of the Umayyad Caliphate under Muawiya I. Ali ibn Abi Talib is generally believed to have been assassinated in Kufa during this year, helping to precipitate the transition of power.

In modern usage, 661 appears as an area code in the North and Central regions of Southern California, including Bakersfield and parts of the Santa Clarita Valley. The 661 area code was created in 1998 as a split from 805.

Altogether, 661 is an example of a prime number with several simple base representations and a notable place in calendar history as the year of Muawiya I's rise and Ali's death.