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"Cardinal  eleven  
"Ordinal  11th (eleventh) 

"Factorization  "prime  
"Prime  5th  
"Divisors  1, 11  
"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} 
11 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 *ainlif, from the prefix *aino (adjectival ""one") and suffix *lif of uncertain meaning.^{[3]} It is sometimes compared with the "Lithuanian vënólika, 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]}^{[4]} it is now sometimes connected with *leiq 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 and German, 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), Japanese 十一 jūichi.
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11 is the 5th smallest "prime number. It is the smallest twodigit prime number in the "decimal "base; as well as, of course, in "undecimal (where it is the smallest twodigit number). It is also the smallest threedigit prime in "ternary, and the smallest fourdigit prime in "binary, but a singledigit prime in bases larger than 11, such as "duodecimal, "hexadecimal, "vigesimal and "sexagesimal. 11 is the fourth "Sophie Germain prime,^{[5]} the third "safe prime,^{[6]} the fourth "Lucas prime,^{[7]} the first "repunit prime,^{[8]} and the second "good prime.^{[9]} 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. 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 3n − 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 × 11 = 1,565,432 → 2,345,651 ÷ 11 = 213,241.)
Because it has a reciprocal of unique period length among primes, 11 is the second "unique prime.^{[10]} 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 11 by onedigit numbers all have matching double digits: 00 (=0), 11, 22, 33, 44, etc. Bob Dorough, in his "Schoolhouse Rock song "The Good Eleven", called them "Doubledigit doogies" (soft g). 11 is the "Aliquot sum of one number, the discrete "semiprime "21 and is the base of the 11aliquot tree.
As 11 is the smallest factor of the first 11 terms of the "Euclid–Mullin sequence, it is the 12th term.^{[11]}
An 11sided "polygon is called a "hendecagon or undecagon.
In both base 6 and base 8, the smallest prime with a composite sum of digits is 11.
Any number b + 1 is written as "11_{b}" in base b, so 11 is trivially a "palindrome in base 10. However 11 is a "strictly nonpalindromic number.^{[12]} It is the only palindromic prime with an even number of digits.
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.^{[13]} 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 10, 11 is the smallest integer that is not a "Nivenmorphic number.
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.
11 is a "Størmer number,^{[14]} a "Heegner number,^{[15]} and a "Mills prime.^{[16]}
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.
The "partition numbers (sequence A000041 in the "OEIS) contain much more multiples of 11 than the oneeleventh one would expect.
According to David A. Klarner, a leading researcher and contributor to the study of "polyominoes, it is possible to cut a rectangle into an odd number of congruent, nonrectangular polyominoes. 11 is the smallest such number, the only such number that is prime, and the only such number that is not a multiple of three.
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.)
"Multiplication  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  50  100  1000  

11 × x  11  "22  "33  "44  "55  "66  "77  "88  "99  "110  "121  "132  "143  "154  "165  "176  "187  "198  "209  "220  "231  "242  "253  "264  "275  550  1100  11000 
"Division  1  2  3  4  5  6  7  8  9  10 

11  12  13  14  15  
11 ÷ x  11  5.5  3.6  2.75  2.2  1.83  1.571428  1.375  1.2  1.1 
1  0.916  0.846153  0.7857142  0.73  
x ÷ 11  0.09  0.18  0.27  0.36  0.45  0.54  0.63  0.72  0.81  0.90 
"1  1.09  1.18  1.27  1.36 
"Exponentiation  1  2  3  4  5  6  7  8  9  10  11  12  13  

11^{x}  11  121  1331  14641  161051  1771561  19487171  214358881  2357947691  25937421601  285311670611  3138428376721  34522712143931  
x^{11}  1  2048  177147  4194304  48828125  362797056  1977326743  8589934592  31381059609  100000000000  285311670611  743008370688  1792160394037 
"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} 
௧௧  "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: "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".^{[17]} A legend that Ursula died with eleven thousand virgin companions ^{[18]} has been thought to appear from misreading XI. M. V. (Latin abbreviation for "Eleven martyr virgins") as "Eleven thousand virgins".
11 is a spiritually significant number in "Thelema.^{["citation needed]}
In the "Enûma Eliš the goddess Tiamat creates eleven monsters to take revenge for the death of her husband, Apsû.
""  Look up eleven in Wiktionary, the free dictionary. 
""  Wikimedia Commons has media related to 11 (number). 