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%I #14 Oct 04 2015 16:12:03
%S 5,4,2,3,4,3,3,4,15,6,4,6,6,3,6,3,4,7,3,7,18,4,6,4,3,7,6,25,7,3,3,4,3,
%T 31,6,4,3,6,3,13,10,12,4,3,13,4,6,3,21,4,43,10,4,9,6,10,7,21,28,19,3,
%U 6,13,4,6,33,7,15,28,19,10,6,18,18,6,21,4,36
%N Smallest k such that (k-1)*prime(n) +/- k are both prime.
%e a(1) = 5 because (5-1)*2 - 5 = 3 and (5-1)*2 + 5 = 13 are both prime;
%e a(2) = 4 because (4-1)*3 - 4 = 5 and (4-1)*3 + 4 = 13 are both prime;
%e a(3) = 2 because (2-1)*5 - 2 = 3 and (2-1)*5 + 2 = 7 are both prime;
%e a(4) = 3 because (3-1)*7 - 3 = 11 and (3-1)*7 + 3 = 17 are both prime;
%e a(5) = 4 because (4-1)*11 - 4 = 29 and (4-1)*11 + 4 = 37 are both prime;
%e a(6) = 3 because (3-1)*13 - 3 = 23 and (3-1)*13 + 3 = 29 are botn prime;
%e a(7) = 3 because (3-1)*17 - 3 = 31 and (3-1)*17 + 3 = 37 are both prime;
%e a(8) = 4 because (4-1)*19 - 4 = 53 and (4-1)*19 + 4 = 61 are both prime;
%e a(9) = 15 because (15-1)*23 - 15 = 307 and (15-1)*23 + 15 = 337 are both prime.
%t sk[n_]:=Module[{k=2},While[!PrimeQ[(k-1)n+k]||!PrimeQ[(k-1)n-k],k++];k]; sk/@Prime[Range[80]] (* _Harvey P. Dale_, Oct 04 2015 *)
%K nonn,less
%O 1,1
%A _Juri-Stepan Gerasimov_, Apr 26 2013
%E Corrected by _R. J. Mathar_, May 04 2013
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