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A077367
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Triangular numbers whose external digits form a triangular number. Or triangular number whose MSD and LSD form a triangular number.
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4
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0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 190, 231, 435, 465, 595, 666, 1035, 1225, 1275, 1485, 1540, 1770, 1830, 2211, 2278, 2628, 2701, 3486, 3916, 4005, 4095, 4465, 5565, 5995, 6216, 6786, 9591, 10440, 10585, 11175, 11325, 11935, 12090
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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MATHEMATICA
| T[n_] := n*(n + 1)/2; tQ[n_] := IntegerQ[Sqrt[8n + 1]]; f2[n_] := Block[{id = IntegerDigits[n]}, If[Length[id] < 3, Return[True]]; tQ[FromDigits[{First[id], Last[id]}]]]; Select[Array[T, 160, 0], f2] (*Chandler*)
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CROSSREFS
| Cf. A077366, A077368.
Sequence in context: A025738 A078256 A025731 * A077366 A077368 A069693
Adjacent sequences: A077364 A077365 A077366 * A077368 A077369 A077370
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KEYWORD
| base,easy,nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Nov 06 2002
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EXTENSIONS
| Extended by Ray Chandler (rayjchandler(AT)sbcglobal.net), Feb 28 2007
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