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Cunningham chain : ウィキペディア英語版
Cunningham chain
In mathematics, a Cunningham chain is a certain sequence of prime numbers. Cunningham chains are named after mathematician A. J. C. Cunningham. They are also called chains of nearly doubled primes.
==Definition==

A Cunningham chain of the first kind of length ''n'' is a sequence of prime numbers (''p''1, ..., ''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).
It follows that
:
\begin
p_2 & = 2p_1+1, \\
p_3 & = 4p_1+3, \\
p_4 & = 8p_1+7, \\
& -1).
\end

Or, by setting a = \frac (the number a is not part of the sequence and need not be a prime number), we have p_i = 2^ a - 1
Similarly, a Cunningham chain of the second kind of length ''n'' is a sequence of prime numbers (''p''1,...,''p''''n'') such that for all 1 ≤ ''i'' < ''n'', ''p''''i''+1 = 2''p''''i'' − 1.
It follows that the general term is
: p_i = 2^p_1 - (2^-1) \,
Now, by setting a = \frac , we have p_i = 2^ a + 1 .
Cunningham chains are also sometimes generalized to sequences of prime numbers (''p''1, ..., ''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 previous or next term in the chain would not be a prime number anymore.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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