Cunningham chain

In mathematics, a Cunningham chain is a certain sequence of prime numbers. Cunningham chains are named after mathematician A. J. C. Cunningham.

A Cunningham chain of the first kind is a sequence of prime numbers (p₁,...,p_n) such that for all 1 ≤ i < n, p_i+1 = 2 p_i + 1. (Hence each term of such a chain except the last one is a Sophie Germain prime, and each term except the first is a safe prime). Similarly, a Cunningham chain of the second kind is a sequence of prime numbers (p₁,...,p_n) such that for all 1 ≤ i < n, p_i+1 = 2 p_i – 1.

Cunningham chains are also sometimes generalized to sequences of prime numbers (p₁,...,p_n) such that for all 1 ≤ i < n, p_i+1 = ap_i + b for fixed coprime integers a, b; the resulting chains are called generalized Cunningham chains.

A Cunningham chain is called complete if it cannot be further extended, i.e., if the next term in the chain would not be a prime number anymore.

External links

• The Prime Glossary: Cunningham chain
• PrimeLinks++: Cunningham chain
• Sequence A005602 in OEIS: the first term of the lowest complete Cunnigham Chains of the first kind of length n, for 1 <= n <= 14
• Sequence A005603 in OEIS: the first term of the lowest complete Cunnigham Chains of the second kind with length n, for 1 <= n <= 15