A241531 - OEIS

%I #10 Apr 27 2014 10:13:58

%S 0,0,1,2,1,1,1,2,5,1,2,3,5,1,1,7,5,1,1,1,1,4,5,1,3,1,1,1,1,3,1,3,3,2,

%T 3,1,3,5,5,5,7,1,1,2,1,2,3,1,1,2,7,3,1,1,3,3,1,2,1,3,1,7,5,5,2,1,6,5,

%U 3,1,3,1,4,1,3,7,5,1,3,1,5,1,7,1,3,13

%N a(n) = smallest k>=1 such that 2*prime(n) - k^2 is semiprime (or 0 if there is no such k).

%e Let n=22, then 2*prime(22) = 2*79 = 158. We have 158-1=157, 158-4=154, 158-9=149, 158-16=142, and only the last number is semiprime. So a(22)=4.

%o (PARI) a(n) = {k = 1; while ((v = 2*prime(n) - k^2) && (v > 0) && (bigomega(v) != 2), k++); if (v <= 0, 0, k);} \\ _Michel Marcus_, Apr 25 2014

%Y Cf. A001358.

%K nonn

%O 1,4

%A _Vladimir Shevelev_, Apr 25 2014

%E More terms from _Michel Marcus_, Apr 25 2014