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A287679
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Practical Pell numbers.
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2
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1, 2, 12, 408, 13860, 470832, 15994428, 543339720, 18457556052, 627013566048, 21300003689580, 723573111879672, 24580185800219268, 835002744095575440, 963592443113182178088, 32733777552734744709300, 1111984844349868137938112, 1283229546787304717998403160
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OFFSET
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1,2
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COMMENTS
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Melfi proved that this sequence is infinite.
The indices of these Pell numbers are 1, 2, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 56, 60, 64, 72, 80, 84, 88, 96, 100, 104, 108, 112, 120, 128, 132, 136, 140, 144, 152, 156, 160, 168, 176, ...
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LINKS
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EXAMPLE
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12 is in this sequence since it is the 4th Pell number, A000129(4) and is also a practical number, A005153(6).
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MATHEMATICA
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PracticalQ[n_] := Module[{f, p, e, prod=1, ok=True}, If[n<1 || (n>1 && OddQ[n]), False, If[n==1, True, f=FactorInteger[n]; {p, e} = Transpose[f]; Do[If[p[[i]] > 1+DivisorSigma[1, prod], ok=False; Break[]]; prod=prod*p[[i]]^e[[i]], {i, Length[p]}]; ok]]]; Select[LinearRecurrence[{2, 1}, {1, 2}, 150], PracticalQ]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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