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 A066569 Triangular numbers whose reverse is also triangular. 5
 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 (list; graph; refs; listen; history; text; internal format)
 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)= { local(d, r=0); while (x>0, d=x%10; x\=10; r=r*10 + d); return(r) } { n=0; for (m=1, 10^10, t=m*(m + 1)/2; if (t%10 == 0, next); if (issquare(8*Rev(t) + 1), write("b066569.txt", n++, " ", t); if (n==100, return)) ) } \\ Harry J. Smith, Mar 08 2010 CROSSREFS Cf. A003098, 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

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Last modified September 16 15:08 EDT 2024. Contains 375976 sequences. (Running on oeis4.)