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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

Table of n, a(n) for n=1..49.

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 May 28 20:12 EDT 2017. Contains 287241 sequences.