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

Author

Vladimir Shevelev, Apr 25 2014

Keywords

• nonn

Examples

			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.
		

Links

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

Crossrefs

Cf. A001358, A241531.

Programs

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