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%I #3 Mar 31 2012 12:38:27
%S 2,3,5,7,11,23,47
%N Primes p with f(p), f(f(p)), ... all prime, where f(n) = (n-1)/2. Stop when f(...f(p)...) is less than 4.
%e (47-1)/2=23(prime);(23-1)/2=11(prime); (11-1)/2=5(prime); (5-1)/2=2.
%t f[n_]:=Module[{k=n},While[k>3,k=(k-1)/2;If[ !PrimeQ[k],Break[]]];PrimeQ[k]]; lst={};Do[p=Prime[n];If[f[p],AppendTo[lst,p]],{n,5!}];lst
%Y Subsequence of A165801.
%K nonn,fini,full
%O 1,1
%A _Vladimir Joseph Stephan Orlovsky_, Sep 28 2009
%E Edited by _Charles R Greathouse IV_, Oct 05 2009