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A077366
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Triangular numbers whose internal digits form a triangular number. Or triangular number such that deleting the MSD and LSD leaves 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, 136, 210, 231, 300, 406, 435, 465, 561, 630, 666, 703, 861, 903, 1035, 2016, 2211, 2556, 3003, 3916, 4005, 5151, 6105, 6216, 6555, 6786, 8001, 8911, 9453, 10011, 10153, 19900, 20100, 20910, 23005
(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]]; f1[n_] := Block[{id = IntegerDigits[n]}, If[Length[id] < 3, Return[True]]; tQ[FromDigits[Take[id, {2, -2}]]]]; Select[Array[T, 250, 0], f1] (*Chandler*)
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CROSSREFS
| Cf. A077367, A077368.
Sequence in context: A078256 A025731 A077367 * A077368 A069693 A069694
Adjacent sequences: A077363 A077364 A077365 * A077367 A077368 A077369
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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
| Corrected and extended by David Wasserman (wasserma(AT)spawar.navy.mil), Apr 28 2003
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