Sophie Germain prime
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 2"n" − 1 to be a Mersenne prime, the converse is not true: 211 − 1 = 2047 which is 23 × 89.