%I #4 Dec 05 2013 19:54:50

%S 1,0,0,0,1,0,0,-1,0,0,4,0,-1,1,0,0,2,-1,0,1,0,0,-1,0,1,1,0,-1,1,0,0,1,

%T -1,0,2,0,0,-1,2,0,1,0,-1,4,0,1,1,-1,0,1,0,0,-1,0,0,2,0,-1,1,3,3,1,-1,

%U 0,1,0,4,-1,0,0,2,2,-1,5,0,0,1,-1,0,4,2,0,-1,0,3,1,0,-1,4,0,0,13

%N a(n) is the smallest number required to make the concatenation of a(n) and 2n+1 a prime, or -1 if no such number exists.

%C a(0) = 0 for a prime.

%e a(9) = 0 as 19 itself is a prime; a(10) = 4 as 421 is a prime but 121, 221, 321 are not.

%K sign,base

%O 0,11

%A _Amarnath Murthy_, Apr 26 2001

%E More terms from _Frank Ellermann_, Jun 15 2001