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%I #16 Sep 09 2014 20:49:30
%S 0,0,0,4,0,6,0,0,9,0,0,12,0,0,15,0,0,18,0,0,21,0,0,0,0,0,0,0,0,30,0,0,
%T 0,0,0,0,0,0,0,0,0,42,0,0,0,0,47,0,0,0,51,0,0,0,0,0,0,0,0,60,0,0,0,0,
%U 0,0,0,0,69,0,0,72,0,0,75,0,0,0,0,0,0,0
%N a(n) = n if A103689(n)*n +/- 1 are twin primes; a(n) = 0 otherwise.
%e For n=5 2*5-1=9 is not prime, 2*5+1=11 is prime so a(5)=0.
%e For n=6 1*6-1=5 is prime, 1*6+1=7 is prime, 5 and 7 are twin primes so a(6)=6.
%t lktp[n_]:=Module[{k=1},While[NoneTrue[k*n+{1,-1},PrimeQ],k++];If[AllTrue[k*n+{1,-1},PrimeQ],n,0]]; Array[lktp,100] (* The program uses the functions NoneTrue and AllTrue from Mathematica version 10 *) (* _Harvey P. Dale_, Sep 09 2014 *)
%o (PARI)
%o a(n)=k=1;while(!isprime(k*n+1)&&!isprime(k*n-1),k++);k
%o for(n=1,100,if(isprime(a(n)*n+1)&&isprime(a(n)*n-1),print1(n,", "));print1(0,", ")) \\ _Derek Orr_, Sep 09 2014
%Y Cf. A103689.
%K easy,nonn
%O 1,4
%A _Pierre CAMI_, Feb 12 2005
%E Corrected by _Harvey P. Dale_, Sep 09 2014
%E Definition simplified by _Derek Orr_, Sep 09 2014