11 (eleven) is the natural number following 10 and preceding 12. Eleven is the smallest positive integer requiring three syllables in English.

<< 10 11 12 13 14 15 16 17 18 19 >> List of numbers -- Integers 0 10 20 30 40 50 60 70 80 90 >>
Cardinal	11 eleven
Ordinal	11th eleventh
Numeral system
Factorization	prime
Divisors	1, 11
Roman numeral	XI
Unicode representation of Roman numeral	Ⅺ, ⅺ
Binary	1011
Hexadecimal	B

In mathematics

Eleven is the 5th smallest prime number, and also a repunit prime. Although it is necessary for n to be prime for 2ⁿ-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.

Because it has a reciprocal of unique period length among primes, 11 is a unique prime. 11 goes into 99 exactly 9 times, so fractions with 11 in the denominator have two digit repeating sequences in their decimal expansions. Multiples of eleven by one-digit numbers all have double digits: 00 (=0), 11, 22, 33, 44 . . .

In base 10, there is a simple test to determine if an integer is divisible by 11: take every digit of the number located in odd position and add them up, then take the remaining digits and add them up. If the difference between the two sums is a multiple of 11, including 0, then the number is divisible by 11.

An eleven-sided polygon is a hendecagon.

In base thirteen and higher bases (such as hexadecimal), eleven is represented as B, where ten is A. In duodecimal, however, eleven is sometimes represented as E and ten as T.

Multiplied by -1, eleven is a Heegner number.

In science

• The atomic number of sodium.

In astronomy,

Messier object M11, a magnitude 7.0 open cluster in the constellation Scutum, also known as the Wild Duck Cluster.

The New General Catalogue object (http://www.ngcic.org/) NGC 11, a spiral galaxy in the constellation Andromeda

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the solar eclipse series which began on -2511 December 26 and ended on -1158 March 18. The duration of Saros series 11 was 1352.2 years, and it contained 76 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -2389 June 19 and ended on -1037 September 8. The duration of Saros series 11 was 1352.2 years, and it contained 76 lunar eclipses.

The 11th moon of Jupiter is Himalia.

In other fields

There are eleven players in a soccer as well as in a cricket team.

The number eleven is important in Numerology, as it is the first of the Master Numbers.

Being only one hour before 12:00 -- midnight -- the eleventh hour means the last possible moment to take care of something, and often implies a situation of urgent danger or emergency (see Doomsday clock).

Uri Geller has always been fascinated by the number eleven. After the September 11 World Trade Center attack, he published an open list of connections to the number eleven involved in that attack.

In Astrology, Aquarius is the 11th astrological sign of the Zodiac.

In the film This Is Spinal Tap, guitarist Nigel Tufnel proudly shows off an amplifier with top settings of 11, higher than the traditional 10.

Eleven is:

• The number of guns in a gun salute to U.S. Army, Air Force and Marine Corps Brigadier Generals, and to Navy and Coast Guard Rear Admirals Lower Half.
• The number of General Orders for Sentries in the Marine Corps.
• The number of spacetime dimensions in M-theory.
• A page in the Service Record Book of an enlisted Marine for writing down disciplinary actions.
• In rugby union the left wing wears the 11 shirt.
• In Internet Explorer, the F11 key toggles full screen viewing mode
• Historical years: 11 A.D., 11 B.C., or 1911
• The number 11 bus is one of the best and cheapest ways of sightseeing in London

da:11 (tal) de:Elf es:Once eo:Dek unu fr:11 (nombre) ia:11 it:Undici la:11 hu:11 (szám) nah:Mahtlahtlince nl:Elf (getal) ja:11 no:11 (tall) pt:Onze sl:11 (število) sv:11 (tal) zh:11