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A259156
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Positive triangular numbers (A000217) that are pentagonal numbers (A000326) divided by 2.
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12
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6, 105, 58311, 1008910, 559902916, 9687554415, 5376187741821, 93019896484620, 51622154137063026, 893177036357767525, 495675918647891434531, 8576285810087387291130, 4759480119234899417304336, 82349495455282056411663435, 45700527609217585557064800441
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OFFSET
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1,1
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COMMENTS
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LINKS
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FORMULA
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G.f.: -x*(x^3+594*x^2+99*x+6) / ((x-1)*(x^2-98*x+1)*(x^2+98*x+1)).
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EXAMPLE
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6 is in the sequence because 6 is the 3rd triangular number, and 2*6 is the 3rd pentagonal number.
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MATHEMATICA
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LinearRecurrence[{1, 9602, -9602, -1, 1}, {6, 105, 58311, 1008910, 559902916}, 20] (* Vincenzo Librandi, Jun 20 2015 *)
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PROG
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(PARI) Vec(-x*(x^3+594*x^2+99*x+6)/((x-1)*(x^2-98*x+1)*(x^2+98*x+1)) + O(x^20))
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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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