11 (number)


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 single-morpheme name.

Name

Eleven derives from the Old English ęndleofon which is first attested in Bede's late 9th-century Ecclesiastical History of the English People. It has cognates in every Germanic language, whose Proto-Germanic ancestor has been reconstructed as *ainalifa-, from the prefix *aina- and suffix *-lifa- of uncertain meaning. It is sometimes compared with the Lithuanian vienúolika, although -lika is used as the suffix for all numbers from 11 to 19.
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 Proto-Germanic *tehun ; it is now sometimes connected with *leikʷ- or *leip-, with the implicit meaning that "one is left" after having already counted to ten.

In languages

While, as mentioned above, 11 has its own name in Germanic languages such as English, German, or Swedish, and some Latin based languages such as Spanish, Portuguese, and French, it is the first compound number in many other languages: Italian ùndici, Chinese 十一 shí yī, Korean 열한 yeol han.

In mathematics

11 is a prime number. It is the smallest two-digit 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.
Multiples of 11 by one-digit numbers all have matching double digits: 00, 11, 22, 33, 44, etc.
An 11-sided 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. For instance, if the number is 65,637 then - = 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 - 637 = -572, which is divisible by 11.
Another test for divisibility is to separate a number into groups of two consecutive 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.
An easy way of multiplying numbers by 11 in base 10 is:
If the number has:
In base 13 and higher bases, 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 in which the 3-variable Helmholtz equation can be solved using the separation of variables technique.
See also 11-cell.
11 of the thirty-five hexominoes can be folded to form cubes. 11 of the sixty-six octiamonds can be folded to form octahedra.
11 is the fourth Sophie Germain prime, the third safe prime, the fourth Lucas prime, the first repunit prime, the second good prime, and the second unique 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.
11 raised to the nth power is the nth row of Pascal's Triangle.
11 is a Heegner number, meaning that the ring of integers of the field has the property of unique factorization.
One consequence of this is that there exists at most one point on the elliptic curve x3 = y2 + 11 that has positive-integer coordinates. In this case, this unique point is.

List of basic calculations

Multiplication123456789101112131415161718192025501001000
11 × x112233445566778899110121132143154165176187198209220275550110011000

Division123456789101112131415
11 ÷ x115.53.2.752.21.81.1.3751.1.110.910.0.70.7
x ÷ 110.0.0.0.0.0.0.0.0.0.11.1.1.1.

Exponentiation12345678910
1111121133114641161051177156119487171214358881235794769125937421601
x120481771474194304488281253627970561977326743858993459231381059609100000000000

Radix151015202530405060708090100
Radix1101201301401502002505001000100001000001000000
x15A1419232837465564738291
x'''A0AA10911812717220841582A75726914A623351--

In numeral systems

In science

Christianity

After Judas Iscariot was disgraced, the remaining apostles of Jesus were sometimes described as "the Eleven" ; this occurred even after Matthias was added to bring the number to twelve, as in Acts 2:14: Peter stood up with the eleven. 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". A legend that Ursula died with eleven thousand virgin companions has been thought to appear from misreading XI. M. V. as "Eleven thousand virgins".

Babylonian

In the Enûma Eliš the goddess Tiamat creates eleven monsters to take revenge for the death of her husband, Apsû.

In music