Chapter 5 - Number Theory and the Real Number System - 5.1 Number Theory: Prime and Composite Numbers - Exercise Set 5.1 - Page 256: 84

11 is a Germain prime

A prime number $p$ is a Germain prime if $2p+1$ is also a prime. This means that 11 is a Germain prime if: $2(11)+1$ is also a prime number. Note that $2(11)+1=23$, which is a prime number. Thus, 11 is a Germain prime.