%I #9 May 02 2018 13:31:31
%S 0,1,3,6,10,15,21,28,36,45,55,66,78,91,105,136,210,231,300,406,435,
%T 465,561,630,666,703,861,903,1035,2016,2211,2556,3003,3916,4005,5151,
%U 6105,6216,6555,6786,8001,8911,9453,10011,10153,19900,20100,20910,23005
%N Triangular numbers whose internal digits form a triangular number. Or triangular number such that deleting the MSD and LSD leaves a triangular number.
%t 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 *)
%Y Cf. A069702, A077367, A077368.
%K base,easy,nonn
%O 1,3
%A _Amarnath Murthy_, Nov 06 2002
%E Corrected and extended by _David Wasserman_, Apr 28 2003