cp's OEIS Frontend

A180303 a(n) = smallest number such that 3^n-2^a(n) is prime, or -1 if no such number exists.

%I A180303 #5 Jul 16 2015 22:05:17
%S A180303 0,1,2,1,1,1,3,3,1,7,4,11,6,3,4,9,9,5,6,3,2,1,7,35,12,29,10,13,6,11,2,
%T A180303 21,8,27,11,-1,1,17,10,-1,1,37,8,9,16,61,23,23,17,27,4,7,2,7,7,39,58,
%U A180303 81,30,17,60,3,8,13,18,-1,20,101,4,73,27,17,2,17,19,13,41,53,44,111,34,13
%N A180303 a(n) = smallest number such that 3^n-2^a(n) is prime, or -1 if no such number exists.
%C A180303 From _Carl R. White_, Oct 23 2010: (Start)
%C A180303 Entries where a(n) = 1 can be found in A014224.
%C A180303 Entries where a(n) = -1 can be found in A181484. (End)
%e A180303 3^1-2^0 = 2, so a(1)=0; no other terms are zero.
%e A180303 3^11-2^1, 3^11-2^2, 3^11-2^3 are all nonprime, but 3^11-2^4 = 177131 which is prime so a(11) = 4.
%e A180303 a(36) is -1 (the placeholder value) because nonprimes are obtained when any power of two is subtracted from 3^36.
%Y A180303 Cf. A013604.
%Y A180303 Cf. A014224, A181483, A181484. - _Carl R. White_, Oct 23 2010
%K A180303 sign
%O A180303 1,3
%A A180303 _Carl R. White_, Aug 25 2010