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A143206
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Product of the n-th cousin prime pair.
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8
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21, 77, 221, 437, 1517, 2021, 4757, 6557, 9797, 11021, 12317, 16637, 27221, 38021, 50621, 53357, 77837, 95477, 99221, 123197, 145157, 159197, 194477, 210677, 216221, 239117, 250997, 378221, 416021, 455621, 549077, 576077, 594437, 680621
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OFFSET
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1,1
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COMMENTS
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Sum_{n>=2} 1/a(n) > 0.02187310784. - R. J. Mathar, Jan 23 2013
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LINKS
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FORMULA
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EXAMPLE
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a(1) = 3*7 = 3*(3+4) = 21;
a(2) = 7*11 = 7*(7+4) = 77;
a(3) = 13*17 = 13*(13+4) = 221;
a(4) = 19*23 = 19*(19+4) = 437.
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MATHEMATICA
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fQ[n_] := Block[{fi = FactorInteger@ n}, Last@# & /@ fi == {1, 1} && Differences[ First@# & /@ fi] == {4}]; Select[ Range@ 700000, fQ] (* Robert G. Wilson v, Feb 08 2012 *)
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PROG
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(Haskell)
a143206 n = a143206_list !! (n-1)
a143206_list = (3*7) : f a000040_list where
f (p:ps@(p':_)) | p'-p == 4 = (p*p') : f ps
| otherwise = f ps
(Magma) [(p*(p+4)): p in PrimesUpTo(1000)| IsPrime(p+4)]; // Vincenzo Librandi, Jan 04 2018
(PARI) lista(nn) = forprime(p=2, nn, if (isprime(q=p+4), print1(p*q, ", "))); \\ Michel Marcus, Jan 04 2018
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CROSSREFS
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KEYWORD
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nonn,changed
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AUTHOR
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STATUS
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approved
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