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a(n) = smallest number such that 3^n-2^a(n) is prime, or -1 if no such number exists.
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%I #5 Jul 16 2015 22:05:17

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

%C From _Carl R. White_, Oct 23 2010: (Start)

%C Entries where a(n) = 1 can be found in A014224.

%C Entries where a(n) = -1 can be found in A181484. (End)

%e 3^1-2^0 = 2, so a(1)=0; no other terms are zero.

%e 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 a(36) is -1 (the placeholder value) because nonprimes are obtained when any power of two is subtracted from 3^36.

%Y Cf. A013604.

%Y Cf. A014224, A181483, A181484. - _Carl R. White_, Oct 23 2010

%K sign

%O 1,3

%A _Carl R. White_, Aug 25 2010