| ||||
---|---|---|---|---|

Cardinal | one hundred twenty-one | |||

Ordinal | 121st (one hundred twenty-first) | |||

Factorization | 11^{2} | |||

Divisors | 1, 11, 121 | |||

Greek numeral | ΡΚΑ�� | |||

Roman numeral | CXXI | |||

Binary | 1111001_{2} | |||

Ternary | 11111_{3} | |||

Quaternary | 1321_{4} | |||

Quinary | 441_{5} | |||

Senary | 321_{6} | |||

Octal | 171_{8} | |||

Duodecimal | A1_{12} | |||

Hexadecimal | 79_{16} | |||

Vigesimal | 61_{20} | |||

Base 36 | 3D_{36} |

**121** (**one hundred [and] twenty-one**) is the natural number following 120 and preceding 122.

## In mathematics

**One hundred [and] 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 , where *p* is prime (3, in this case). Other such squares must have at least 35 digits.

There are only two other squares known to be of the form n! + 1, supporting Brocard's conjecture. Another example of 121 being of the few examples supporting a conjecture is that Fermat conjectured that 4 and 121 are the only perfect squares of the form *x*^{3} - 4 (with *x* being 2 and 5, respectively).^{[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 (11^2). But it can not be expressed as the sum of any other number plus that number's digits, making 121 a self number.

## In other fields

121 is also:

- The electricity emergency telephone number in Egypt
- The number for voicemail for mobile phones on the Vodafone network
^{[2]} - The undiscovered chemical element Unbiunium has the atomic number 121
- The official end score for Cribbage
^{[3]}

## See also

- List of highways numbered 121
- United States House of Representatives House Resolution 121
- United Nations Security Council Resolution 121

## References

**^**Wells, D.,*The Penguin Dictionary of Curious and Interesting Numbers*, London: Penguin Group. (1987): 136**^**Vodafone, Calling and messaging**^**Rule 1.1, American Cribbage Congress, retrieved 6 September 2011