121 (number)

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← 120 121 122 →
Cardinalone hundred twenty-one
(one hundred twenty-first)
Divisors1, 11, 121
Greek numeralΡΚΑ´
Roman numeralCXXI

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

In mathematics[edit]

One hundred [and] twenty-one' is a square (11 times 11) 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 one of the few numbers supporting a conjecture is that Fermat conjectured that 4 and 121 are the only perfect squares of the form x3 - 4 (with x being 2 and 5, respectively).[1] It is also a star number and a centered octagonal number.

A Chinese checkers board has 121 holes

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[edit]

121 is also:

See also[edit]


  1. ^ Wells, D., The Penguin Dictionary of Curious and Interesting Numbers, London: Penguin Group. (1987): 136
  2. ^ Vodafone, Calling and messaging
  3. ^ Rule 1.1 Archived 2015-01-18 at the Wayback Machine, American Cribbage Congress, retrieved 6 September 2011