A241531 a(n) = smallest k>=1 such that 2*prime(n) - k^2 is semiprime (or 0 if there is no such k).

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, 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, 3, 1, 3, 1, 4, 1, 3, 7, 5, 1, 3, 1, 5, 1, 7, 1, 3, 13

Offset: 1

Author

Vladimir Shevelev, Apr 25 2014

Keywords

• nonn

Examples

			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.
		

Crossrefs

Cf. A001358.

Programs

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

Extensions

More terms from Michel Marcus, Apr 25 2014