This article is about the number. For other uses, see 11 (disambiguation).
"Ⅺ" redirects here. For other uses, see XI (disambiguation) and xi (disambiguation).
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Cardinal  eleven  
Ordinal  11th (eleventh)  
Factorization  prime  
Prime  5th  
Divisors  1, 11  
Greek numeral  ΙΑ´  
Roman numeral  XI  
Greek prefix  hendeca/hendeka  
Latin prefix  undeca  
Binary  1011_{2}  
Ternary  102_{3}  
Quaternary  23_{4}  
Quinary  21_{5}  
Senary  15_{6}  
Octal  13_{8}  
Duodecimal  B_{12}  
Hexadecimal  B_{16}  
Vigesimal  B_{20}  
Base 36  B_{36}  
Hebrew numeral  יא 
11 (eleven) is the natural number following 10 and preceding 12. It is the first repdigit. In English, it is the smallest positive integer requiring three syllables and the largest prime number with a singlemorpheme name.
Eleven derives from the Old English ęndleofon which is first attested in Bede's late 9thcentury Ecclesiastical History of the English People.^{[2]}^{[3]} It has cognates in every Germanic language (for example, German elf), whose ProtoGermanic ancestor has been reconstructed as *ainalifa,^{[4]} from the prefix *aina (adjectival "one") and suffix *lifa of uncertain meaning.^{[3]} It is sometimes compared with the Lithuanian vienúolika, although lika is used as the suffix for all numbers from 11 to 19 (analogous to "teen").^{[3]}
The Old English form has closer cognates in Old Frisian, Saxon, and Norse, whose ancestor has been reconstructed as *ainlifun. This has formerly been considered derived from ProtoGermanic *tehun ("ten");^{[3]}^{[5]} it is now sometimes connected with *leikʷ or *leip ("left; remaining"), with the implicit meaning that "one is left" after having already counted to ten.^{[3]}
While, as mentioned above, 11 has its own name in Germanic languages such as English, German, and Swedish. It is the first compound number in many other languages, e.g., Italian ùndici (but in Spanish and Portuguese, 16, and in French, 17 is the first compound number), Chinese 十一 shí yī, Korean 열한 yeol han.
11 is a prime number. It is the smallest twodigit prime number in the decimal base.
The next prime is 13, with which it comprises a twin prime.
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 × 11 = 1,565,432 → 2,345,651 ÷ 11 = 213,241.)
Multiples of 11 by onedigit numbers all have matching double digits: 00 (=0), 11, 22, 33, 44, etc.
An 11sided polygon is called a hendecagon or undecagon.
There are 11 regular and semiregular convex uniform tilings in the second dimension, and 11 planigons that correspond to these 11 regular and semiregular tilings.
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.^{[6]} For instance, if the number is 65,637 then (6 + 6 + 7)  (5 + 3) = 19  8 = 11, so 65,637 is divisible by 11. This technique also works with groups of digits rather than individual digits, so long as the number of digits in each group is odd, although not all groups have to have the same number of digits. For instance, if one uses three digits in each group, one gets from 65,637 the calculation (065)  637 = 572, which is divisible by 11.
Another test for divisibility is to separate a number into groups of two consecutive digits (adding a leading zero if there is an odd number of digits), and then add up the numbers so formed; if the result is divisible by 11, the number is divisible by 11. For instance, if the number is 65,637, 06 + 56 + 37 = 99, which is divisible by 11, so 65,637 is divisible by eleven. This also works by adding a trailing zero instead of a leading one: 65 + 63 + 70 = 198, which is divisible by 11. This also works with larger groups of digits, providing that each group has an even number of digits (not all groups have to have the same number of digits).
An easy way of multiplying numbers by 11 in base 10 is: If the number has:
In base 13 and higher bases (such as hexadecimal), 11 is represented as B, where ten is A. In duodecimal, however, 11 is sometimes represented as E and ten as T or X.
There are 11 orthogonal curvilinear coordinate systems (to within a conformal symmetry) in which the 3variable Helmholtz equation can be solved using the separation of variables technique.
See also 11cell.
11 of the thirtyfive hexominoes can be folded to form cubes. 11 of the sixtysix octiamonds can be folded to form octahedra.
11 is the fourth Sophie Germain prime,^{[7]} the third safe prime,^{[8]} the fourth Lucas prime,^{[9]} the first repunit prime,^{[10]} the second good prime,^{[11]} and the second unique prime.^{[12]} Although it is necessary for n to be prime for 2^{n} − 1 to be a Mersenne prime, the converse is not true: 2^{11} − 1 = 2047 which is 23 × 89.
11 raised to the nth power is the nth row of Pascal's Triangle. (This works for any base, but the number eleven must be changed to the number represented as 11 in that base; for example, in duodecimal this must be done using thirteen.)
11 is a Heegner number, meaning that the ring of integers of the field has the property of unique factorization.
Exponentiation  1  2  3  4  5  6  7  8  9  10 

11^{x}  11  121  1331  14641  161051  1771561  19487171  214358881  2357947691  25937421601 
x^{11}  1  2048  177147  4194304  48828125  362797056  1977326743  8589934592  31381059609  100000000000 
Radix  1  5  10  15  20  25  30  40  50  60  70  80  90  100 

110  120  130  140  150  200  250  500  1000  10000  100000  1000000  
x_{11}  1  5  A_{11}  14_{11}  19_{11}  23_{11}  28_{11}  37_{11}  46_{11}  55_{11}  64_{11}  73_{11}  82_{11}  91_{11} 
A0_{11}  AA_{11}  109_{11}  118_{11}  127_{11}  172_{11}  208_{11}  415_{11}  82A_{11}  7572_{11}  6914A_{11}  623351_{11} 
११  Devanagari 

கக  Tamil 
൧൧  Malayalam 
౧౧  Telugu 
১১  Bangla 
After Judas Iscariot was disgraced, the remaining apostles of Jesus were sometimes described as "the Eleven" (Mark 16:11; Luke 24:9 and 24:33); this occurred even after Matthias was added to bring the number to twelve, as in Acts 2:14:^{[14]} Peter stood up with the eleven (New International Version). The New Living Translation says Peter stepped forward with the eleven other apostles, making clear that the number of apostles was now twelve.
Saint Ursula is said to have been martyred in the third or fourth century in Cologne with a number of companions, whose reported number "varies from five to eleven".^{[15]} A legend that Ursula died with eleven thousand virgin companions^{[16]} has been thought to appear from misreading XI. M. V. (Latin abbreviation for "Eleven martyr virgins") as "Eleven thousand virgins".
In the Enûma Eliš the goddess Tiamat creates eleven monsters to take revenge for the death of her husband, Apsû.
See also: Eleven (disambiguation) § Music
Look up eleven in Wiktionary, the free dictionary. 
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