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A241809
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Semiprimes sp such that sp+2 is a prime.
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3
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9, 15, 21, 35, 39, 51, 57, 65, 69, 77, 87, 95, 111, 129, 155, 161, 177, 209, 221, 237, 249, 267, 291, 305, 309, 329, 335, 365, 371, 377, 381, 395, 407, 417, 437, 447, 485, 489, 497, 501, 519, 545, 591, 597, 611, 629, 671, 681, 689, 699, 707, 717, 731, 737, 749
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OFFSET
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1,1
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LINKS
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FORMULA
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EXAMPLE
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a(2) = 15 = 3*5, which is semiprime and 15+2 = 17 is a prime.
a(6) = 51 = 3*17, which is semiprime and 51+2 = 53 is a prime.
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MAPLE
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with(numtheory): A241809:= proc(); if bigomega(x)=2 and isprime(x+2)then RETURN (x); fi; end: seq(A241809 (), x=1..2000);
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MATHEMATICA
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Select[Prime[Range[200]]-2, PrimeOmega[#]==2&] (* Harvey P. Dale, Aug 06 2015 *)
SequencePosition[Table[Which[PrimeQ[n], 1, PrimeOmega[n]==2, 2, True, 0], {n, 800}], {2, _, 1}][[;; , 1]] (* Harvey P. Dale, Oct 05 2023 *)
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PROG
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(PARI) for(k=1, 1000, if(bigomega(k)==2 && isprime(k+2), print1(k, ", "))) \\ Colin Barker, May 07 2014
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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