A241533 Smallest prime p such that 2*prime(n) - p^2 is semiprime, or a(n)=0 if there is no such p.

0, 0, 2, 2, 0, 2, 3, 2, 5, 3, 2, 3, 5, 2, 3, 7, 5, 2, 7, 3, 2, 5, 5, 3, 3, 5, 2, 3, 2, 3, 7, 3, 3, 2, 3, 2, 3, 5, 5, 5, 7, 2, 13, 2, 19, 2, 3, 3, 3, 2, 7, 3, 2, 3, 3, 3, 3, 2, 3, 3, 2, 7, 5, 5, 2, 0, 13, 5, 3, 2, 3, 7, 7, 3, 3, 7, 5, 3, 3, 5, 5, 2, 7, 2, 3, 13

Offset

1,3

Links

Peter J. C. Moses, Table of n, a(n) for n = 1..1000

Example

Let n=16, then 2*prime(16) = 2*53 = 106. We have 106-4=102, 106-9=97, 106-25=81, 106-49=57, and only the last number is semiprime. So a(16)=7.

Prog

(PARI) a(n) = {for (i=1, n, if ((v = 2*prime(n) - prime(i)^2) <= 0, break; ); if (bigomega(v) == 2, return (prime(i))); ); } \\ Michel Marcus, May 09 2014

Crossrefs

Cf. A001358, A241531.

Sequence in context: A361869 A256750 A228430 * A370885 A072738 A165316

Adjacent sequences: A241530 A241531 A241532 * A241534 A241535 A241536

Keyword

nonn

Author

Vladimir Shevelev, Apr 25 2014

Status

approved