OFFSET
1,1
COMMENTS
Primes p such that {(p-1)/2, p, 2p+1, 4p+3, 8p+7, 16p+15, 32p+31} = {nonprime, prime, prime, prime, prime, prime, composite}.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
Chris Caldwell's Prime Glossary, Cunningham chains.
EXAMPLE
2 is here because (2-1)/2 = 1/2 and 32*2+31 = 95 are not primes, while 2, 5, 11, 23, and 47 give a first-kind Cunningham chain of 5 primes which cannot be continued.
53639 is here because through <2p+1>, 53639 -> 107279 -> 214559 -> 429119 -> 858239 and the chain ends here (with this operator).
MATHEMATICA
l5Q[n_]:=Module[{a=PrimeQ[(n-1)/2], b=PrimeQ[ NestList[2#+1&, n, 5]]}, Join[{a}, b]=={False, True, True, True, True, True, False}]; Select[Range[ 1300000], l5Q] (* Harvey P. Dale, Oct 14 2012 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Feb 20 2001
EXTENSIONS
Definition corrected by Alexandre Wajnberg, Aug 31 2005
Entry revised by N. J. A. Sloane, Apr 01 2006
STATUS
approved