OFFSET
1,3
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
KEYWORD
base,easy,nonn
AUTHOR
Amarnath Murthy, Nov 06 2002
EXTENSIONS
Corrected and extended by David Wasserman, Apr 28 2003
STATUS
approved