# ELEVEN

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Eleven is the first number which cannot be counted with a human's ten fingers. In English, it is the smallest positive integer requiring three syllables and the largest prime number with a single-morpheme name. Its etymology originates from a Germanic compound ainlif meaning "one left".

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 x 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. 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 matching double digits: 00 (=0), 11, 22, 33, 44, etc. Bob Dorough, in his Schoolhouse Rock song "The Good Eleven", called them "Double-digit doogies" (soft g). Eleven is the Aliquot sum of one number, the discrete semiprime 21 and is the base of the 11-aliquot tree.

Eleven is the atomic number of sodium. 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.

Computers of the PDP-11 series from Digital Equipment Corporation were informally referred to as "elevens". The number 11 bus is a low-cost way of sightseeing in London. It goes from Liverpool St to Fulham Broadway via the West End.

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