# 11 number

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

Template:Numbers 10s
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

## In mathematics

Eleven is the 5th smallest prime number. 11 is the fourth Sophie Germain prime, the third safe prime, the fourth Lucas prime, and the first 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. 11 is an Eisenstein prime with no imaginary part and real part of the form ${\displaystyle 3n-1}$.

Because it has a reciprocal of unique period length among primes, 11 is the second unique prime. 11 goes into 99 exactly 9 times, so vulgar 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.

In base 10, there is also an easy way of multiplying numbers by 11. If the number has:

• 1 digit - Replicate the digit (so 2 x 11 becomes 22).
• 2 digits - Add the 2 digits together and place the result in the middle (so 47 x 11 becomes 4 (11) 7=4 (1+10) 7=(4+1) 1 7=517).

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 Template:Num/neg, eleven is a Heegner number.

## In science

In astronomy,

The approximate periodicity is 11 years in sunspot cycle.
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 NGC 11, a spiral galaxy in the constellation Andromeda
The Saros number 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 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. Within a school or college, the phrases "the football XI" and "the cricket XI" generally refer to the first (best) team currently playing. Other teams are often referred to as "the second XI" etc.

An American football team also has eleven players.

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.

The quote "Eleven! That's ridiculous, it's not even funny" from Negativland composition "Time Zones" (referring to the number of time zones in USSR) is referenced many times in multiplayer online game Kingdom of Loathing. As a result many KoL players say that phrase every time they encounter number 11 (often without knowing its true origins).

Ocean's Eleven is the name of two American films.

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 both Mozilla Firefox and Internet Explorer for Windows, the F11 key toggles full screen viewing mode. In Mac OS X, the F11 key hides all open windows.
• 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
• The number of thumb keys on a bassoon, not counting the whisper key. (A few bassons have a twelfth thumb key).