Eleven (11) is a positive integer one more than 10 and one less than 12.

## Properties

11 is the fifth prime number (between 7 and 13). It is a member of a twin prime pair with 13.

11 is the first prime that isn't a Mersenne exponent, since 2^{11}-1 = [[2047] = 23*89.

11 is the fifth Lucas number, between 7 and 18.

## In googology

In Greek-based number-naming systems, is associated with prefix "**endeka-**", and with prefix "**undec-**" in Latin systems.

Wiki user Denis Maksudov calls this number **zeralum**, and it's equal to f^{0}(10) using the fast-growing hierarchy.^{[1]}

There are several methods to test for divisibility by 11. The easiest is to alternately add and subtract digits, starting from the left, and starting at a subtraction. So 429 is aa multiple of 11 since 4-2+9 = 11. Another way is to add groups of two digits, starting from the right.

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