A066569 Triangular numbers whose reverse is also triangular.

1, 3, 6, 55, 66, 153, 171, 351, 595, 666, 3003, 5995, 8778, 15051, 17578, 66066, 87571, 185745, 547581, 617716, 828828, 1269621, 1461195, 1680861, 3544453, 5073705, 5676765, 5911641, 6295926, 12145056, 12517506, 35133153, 60571521

Offset

1,2

Comments

Numbers ending in 0 are not included. - Harry J. Smith, Mar 06 2010

Links

Harry J. Smith, Table of n, a(n) for n = 1..100

Example

153 and 351 are both triangular.

Mathematica

dtn[L_] := Fold[10#1+#2&, 0, L] tritest[n_] := Module[{t}, t=Floor[N[Sqrt[2n]]]; 2n==t(t+1)] A={}; For[i=1, i>0, i++, t=i(i+1)/2; If[tritest[dtn[Reverse[IntegerDigits[t]]]]&&Mod[t, 10]>0, AppendTo[A, t]; Print[A]]]
Select[Accumulate[Range[12000]], Last[IntegerDigits[#]]!=0&&OddQ[Sqrt[1+ 8*FromDigits[Reverse[IntegerDigits[#]]]]]&] (* Harvey P. Dale, Jun 04 2015 *)

Prog

(PARI)
Rev(x)= { fromdigits(Vecrev(digits(x))) }
{ for (m=1, 10^6, my(t=m*(m + 1)/2); if (t%10 && issquare(8*Rev(t) + 1), print1(t, ", ")) ) } \\ Harry J. Smith, Mar 08 2010

Crossrefs

Cf. A000217, A003098, A004086, A061455, A066528, A069673.

Sequence in context: A132474 A032070 A355637 * A051641 A003098 A045914

Adjacent sequences: A066566 A066567 A066568 * A066570 A066571 A066572

Keyword

base,nonn

Author

Erich Friedman, Jan 08 2002

Extensions

Offset changed from 0 to 1 by Harry J. Smith, Mar 06 2010

Status

approved