

A241533


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


4



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
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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 1064=102, 1069=97, 10625=81, 10649=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: A099307 A256750 A228430 * A072738 A165316 A215976
Adjacent sequences: A241530 A241531 A241532 * A241534 A241535 A241536


KEYWORD

nonn


AUTHOR

Vladimir Shevelev, Apr 25 2014


STATUS

approved



