Sophie Germain prime
A Cunningham chain of the first kind of length "n" is a sequence of prime numbers ("p"1, ..., "p""n") such that "p""i"+1 = 2"p""i" + 1 for all 1 ≤ "i" &lt; "n". (Hence each term of such a chain except the last is a Sophie Germain prime, and each term except the first is a safe prime).