121 (number)
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121 is the natural number following 120 and preceding 122.
| Template:Numbers 120s | |
| Cardinal | One hundred [and] twenty-one |
| Ordinal | 121st |
| Factorization | <math>11^2<math> |
| 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 <math>1 + p + p^2 + p^3 + p^4<math>, 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.