121 number

121 is the natural number following 120 and preceding 122.

Template:Numbers 120s
Cardinal	One hundred [and] twenty-one
Ordinal	121st
Factorization	$11^{2}$
Roman numeral	CXXI
Binary	1111001
Hexadecimal	79

One hundred twenty-one is a square and is the sum of three consecutive primes (37 + 41 + 43). There are no squares besides 121 known to be of the form $1+p+p^{2}+p^{3}+p^{4}$, where p is prime (3, in this case). There are only two other squares known to be of the form n! + 1.

It is also a star number and a centered octagonal number.

In base 10, it is a Smith number since its digits add up to the same value as its factorization (which uses the same digits) and as a consequence of that it is a Friedman number. But it can not be expressed as the sum of any other number plus that number's digits, making 121 a self number.

One hundred twenty-one is also:

• The number for voicemail for mobile phones on the Vodafone network
• The model number of the Lockheed EC-121, now on display at the Air Force Museum.
• The year AD 121 or 121 BC.