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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 (list; graph; refs; listen; history; text; internal format)
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: A099307 A256750 A228430 * A072738 A165316 A215976

Adjacent sequences:  A241530 A241531 A241532 * A241534 A241535 A241536

KEYWORD

nonn

AUTHOR

Vladimir Shevelev, Apr 25 2014

STATUS

approved

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Last modified April 19 08:53 EDT 2021. Contains 343110 sequences. (Running on oeis4.)