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A066757
Pentagonal numbers not divisible by 10 whose reverse is triangular.
0
0, 1, 12, 51, 18426, 69876, 6168162, 12105501, 17759001, 3008521945, 100938635992, 826471253301, 3044088035830227, 5889438558016867, 55959294872171901
OFFSET
1,3
EXAMPLE
51 is pentagonal and 15 is triangular
MATHEMATICA
dtn[L_] := Fold[10#1+#2&, 0, L] tritest[n_] := Module[{t}, t=Floor[N[Sqrt[2n]]]; 2n==t(t+1)] A={0}; For[i=1, i>0, i++, t=i(3i-1)/2; r=dtn[Reverse[IntegerDigits[t]]]; If[tritest[r]&&Mod[t, 10]>0, AppendTo[A, t]; Print[A]]]
Select[Table[(n(3n-1))/2, {n, 0, 194*10^6}], Mod[IntegerReverse[#], 10]!=0&&OddQ[Sqrt[8IntegerReverse[#]+1]]&] (* Requires Mathematica version 10 or later *) (* The program will take a long time to run *) (* Harvey P. Dale, Sep 12 2017 *)
CROSSREFS
Sequence in context: A115680 A231298 A231359 * A225306 A331041 A317466
KEYWORD
base,more,nonn
AUTHOR
Erich Friedman, Jan 17 2002
EXTENSIONS
a(13)-a(15) from Lars Blomberg, May 18 2011
Description clarified and offset corrected by Lars Blomberg, May 29 2011
STATUS
approved