Eleven is a number. In English, it is the smallest positive integer requiring three syllables and the largest prime number with a single-morpheme name. Its etymology originates from a Germanic compound
ainlif meaning "one left".
11 is the 5th smallest prime number. It is the smallest two-digit prime number in the decimal base; as well as, of course, in undecimal (where it is the smallest two-digit number). It is also the smallest three-digit prime in ternary, and the smallest four-digit prime in binary, but a single-digit prime in bases larger than 11, such as duodecimal, hexadecimal, vigesimal and sexagesimal. 11 is the fourth Sophie Germain prime, the third safe prime, the fourth Lucas prime, the first repunit prime, and the second good prime. Although it is necessary for
n to be prime for 2
n − 1 to be a Mersenne prime, the converse is not true: 211 − 1 = 2047 which is 23 × 89. The next prime is 13, with which it comprises a twin prime. 11 is an Eisenstein with no imaginary part and real part of the form 3
n − 1. Displayed on a calculator, 11 is a strobogrammatic prime and a dihedral prime because it reads the same whether the calculator is turned upside down or reflected on a mirror, or both.
If a number is divisible by 11, reversing its digits will result in another multiple of 11. As long as no two adjacent digits of a number added together exceed 9, then multiplying the number by 11, reversing the digits of the product, and dividing that new number by 11, will yield a number that is the reverse of the original number. (For example: 142,312 x 11 = 1,565,432. 2,345,651 / 11 = 213,241.)
11 Downing Street is the official residence of the Chancellor of the Exchequer.
11 Americans fought in the RAF in the Battle of Britain, one half of one percent of all those who fought in the RAF in the battle.
The Eagles Squadron numbered just 250 personnel - a thousandth of the entire manpower of the RAF - and lasted just over a year, before it left the RAF and joined the United States Army Air Force when the US entered the war.
Foreigners saved the brits azzes once again.
Under 20% of those who fought in the RAF in WWII were foreigners.