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Search: id:A112777
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| A112777 |
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Numbers n such that 2*n^2 + 1 is a semiprime. |
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+0
3
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| 2, 4, 5, 8, 10, 12, 13, 14, 15, 17, 18, 19, 23, 28, 31, 32, 39, 44, 48, 49, 50, 53, 54, 55, 57, 58, 60, 63, 64, 68, 69, 71, 76, 78, 81, 82, 84, 85, 86, 89, 90, 91, 104, 108, 111, 112, 113, 116, 118, 120, 122, 126, 127, 129, 134, 138, 141, 143, 144, 147, 150, 157, 159, 163
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Primes in this sequence include: 2, 5, 13, 17, 19, 23, 31, 53, 71, 89, 113, 127, 157, 159, 163, 167, 181, 197, 229. Semiprimes in this sequence include: 4, 10, 14, 15, 39, 49, 55, 57, 58, 69, 82, 85, 86, 91, 111, 118, 122, 129, 134, 141, 143, 166, 183, 185, 215, 217, 221, 249. - Jonathan Vos Post (jvospost3(AT)gmail.com), Nov 12 2005
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FORMULA
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a(n) = Sqrt(A111350(n)).
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MATHEMATICA
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Select[Range[165], Plus @@ Last /@ FactorInteger[2#^2 + 1] == 2 &]
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CROSSREFS
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Cf. A111350, A111351.
Sequence in context: A067938 A018457 A046809 this_sequence A047612 A123886 A005242
Adjacent sequences: A112774 A112775 A112776 this_sequence A112778 A112779 A112780
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KEYWORD
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nonn
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AUTHOR
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Ray Chandler (rayjchandler(AT)sbcglobal.net), Nov 10 2005
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