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A113519
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Semiprimes in first spoke of a hexagonal spiral (A056105).
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1
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9, 22, 134, 177, 226, 482, 737, 1046, 1282, 1681, 1977, 2641, 3202, 3401, 3817, 4034, 4486, 5462, 5721, 6817, 7401, 7702, 8966, 9634, 9977, 10681, 11042, 11409, 12937, 15409, 16726, 17177, 18566, 21506, 28617, 29801
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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FORMULA
| {a(n)} = {3*n^2 - 2*n + 1 iff semiprime}. {a(n)} = A056105 INTERSECT A001358.
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EXAMPLE
| A056105(44) = 3*44^2 - 2*44 + 1 = 5721 = 3 * 1907 which is a semiprime.
A056105(24) = 3*24^2 - 2*24 + 1 = 1681 = 41^2 which is a semiprime (the two prime factors need not be distinct).
A056105(100) = 3*100^2 - 2*100 + 1 = 29801 = 17 * 1753, which is a semiprime.
41 is not in this sequence because A056105(4) = 41 is prime.
3009 is not in this sequence because A056105(32) = 3009 = 3 * 17 * 59 is a 3-almost prime.
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CROSSREFS
| Cf. A001358, A056105.
Sequence in context: A049730 A131895 A197498 * A123833 A084023 A156342
Adjacent sequences: A113516 A113517 A113518 * A113520 A113521 A113522
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KEYWORD
| easy,nonn
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Jan 12 2006
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