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 A077367 Triangular numbers whose external digits form a triangular number. Or triangular number whose MSD and LSD form a triangular number. 4
 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; text; internal format)
 OFFSET 1,3 LINKS 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] (* Ray Chandler, Feb 28 2007 *) CROSSREFS Cf. A077366, A077368. Sequence in context: A025738 A078256 A025731 * A077366 A077368 A069693 Adjacent sequences:  A077364 A077365 A077366 * A077368 A077369 A077370 KEYWORD base,easy,nonn AUTHOR Amarnath Murthy, Nov 06 2002 EXTENSIONS Extended by Ray Chandler, Feb 28 2007 STATUS approved

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Last modified October 18 02:23 EDT 2019. Contains 328135 sequences. (Running on oeis4.)