A077367 Triangular numbers whose external digits form a triangular number. Or triangular number whose MSD and LSD form a triangular number.

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

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