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A069499
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Triangular numbers of the form 21*k.
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1
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0, 21, 105, 210, 231, 378, 630, 861, 903, 1176, 1596, 1953, 2016, 2415, 3003, 3486, 3570, 4095, 4851, 5460, 5565, 6216, 7140, 7875, 8001, 8778, 9870, 10731, 10878, 11781, 13041, 14028, 14196, 15225, 16653, 17766, 17955, 19110, 20706, 21945, 22155, 23436, 25200
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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G.f.: -21*x^2*(x^2-x+1)*(x^4+5*x^3+9*x^2+5*x+1) / ((x-1)^3*(x+1)^2*(x^2+1)^2). - Colin Barker, Sep 23 2013
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MAPLE
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a[0] := 0:a[1] := 6:a[2] := 14:a[3] := 20:a[4] := 21:a[5] := 27:a[6] := 35:a[7] := 41:seq((42*(floor(i/8))+a[i mod 8])*(42*(floor(i/8))+a[i mod 8]+1)/2, i=0..100);
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MATHEMATICA
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Select[21Range[1100], OddQ[Sqrt[8#+1]]&] (* Harvey P. Dale, Aug 16 2021 *)
Select[Accumulate[Range[0, 300]], IntegerQ[#/21]&] (* Harvey P. Dale, Jun 12 2022 *)
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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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EXTENSIONS
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STATUS
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approved
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