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

1,4

Links

Table of n, a(n) for n=1..86.

Example

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.

Prog

(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

Crossrefs

Cf. A001358.

Sequence in context: A061545 A287641 A265312 * A362277 A367955 A273894

Adjacent sequences: A241528 A241529 A241530 * A241532 A241533 A241534

Keyword

nonn

Author

Vladimir Shevelev, Apr 25 2014

Extensions

More terms from Michel Marcus, Apr 25 2014

Status

approved