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A116544
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Triangular numbers for which the multiplicative digital root is also a triangular number.
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1
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0, 1, 3, 6, 10, 28, 45, 55, 78, 105, 120, 190, 210, 231, 253, 276, 300, 325, 378, 406, 435, 465, 528, 561, 595, 630, 703, 741, 780, 820, 861, 903, 990, 1035, 1081, 1128, 1225, 1275, 1378, 1485, 1540, 1596, 1653, 1770, 1830, 2016, 2080, 2145, 2278, 2346, 2415
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OFFSET
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1,3
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LINKS
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MATHEMATICA
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a = {}; For[n = 0, n < 100, n++, b = n*(n + 1)/2; While[b > 9, b = Times @@ IntegerDigits[b]]; If[Length[Intersection[{b}, {0, 1, 3, 6}]] == 1, AppendTo[a, n*(n + 1)/2]]]; a (* Stefan Steinerberger, Jan 02 2008 *)
dr[n_]:=NestWhile[Times@@IntegerDigits[#]&, n, #>9&]; With[{trnos= Accumulate[Range[0, 80]]}, Select[trnos, MemberQ[trnos, dr[#]]&]] (* Harvey P. Dale, Nov 02 2011 *)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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Luc Stevens (lms022(AT)yahoo.com), Apr 03 2006
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EXTENSIONS
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STATUS
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approved
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