cp's OEIS Frontend

A282194 a(n) = smallest positive k such that 2*n + 2^k + 1 is composite.

%I A282194 #21 Apr 26 2017 13:21:46
%S A282194 3,5,2,1,4,2,1,7,2,1,2,1,1,3,2,1,1,2,1,4,2,1,2,1,1,2,1,1,3,2,1,1,2,1,
%T A282194 3,2,1,1,2,1,2,1,1,2,1,1,1,2,1,4,2,1,4,2,1,2,1,1,1,1,1,1,2,1,2,1,1,3,
%U A282194 2,1,1,1,1,3,2,1,1,2,1,1,2,1,2,1,1,2,1,1,3,2,1,1,1,1,4,2,1,3,2,1,1,1,1,1,2,1,1,1,1
%N A282194 a(n) = smallest positive k such that 2*n + 2^k + 1 is composite.
%C A282194 Least k such that a(k) = n are 3, 2, 0, 4, 1, 112, 7, 32917, 802, 9712, 1198673602 for the initial terms.
%H A282194 Altug Alkan, <a href="/A282194/b282194.txt">Table of n, a(n) for n = 0..10000</a>
%e A282194 a(1) = 5 because 3 + 2^k is prime for 0 < k < 5 and 3 + 2^5 = 35 is composite.
%t A282194 spk[n_]:=Module[{k=1},While[!CompositeQ[2n+2^k+1],k++];k]; Array[spk,110,0] (* _Harvey P. Dale_, Apr 26 2017 *)
%o A282194 (PARI) a(n) = my(k=1); while(isprime(2*n+2^k+1), k++); k;
%Y A282194 Cf. A067076, A067760, A153238, A282026.
%K A282194 nonn
%O A282194 0,1
%A A282194 _Altug Alkan_, Feb 15 2017