A077366 Triangular numbers whose internal digits form a triangular number. Or triangular number such that deleting the MSD and LSD leaves a triangular number.

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

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]]; f1[n_] := Block[{id = IntegerDigits[n]}, If[Length[id] < 3, Return[True]]; tQ[FromDigits[Take[id, {2, -2}]]]]; Select[Array[T, 250, 0], f1] (* Ray Chandler, Feb 28 2007 *)

Crossrefs

Cf. A069702, A077367, A077368.

Sequence in context: A078256 A025731 A077367 * A077368 A069693 A069694

Adjacent sequences: A077363 A077364 A077365 * A077367 A077368 A077369

Keyword

base,easy,nonn

Author

Amarnath Murthy, Nov 06 2002

Extensions

Corrected and extended by David Wasserman, Apr 28 2003

Status

approved