11

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.

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.