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Triangular numbers composed of digits {1,3,5}.
2

%I #20 Mar 31 2023 00:51:00

%S 1,3,15,55,153,351,5151,33153,35511,3531153,3555111,35133153,51151555,

%T 51313515,355311153,355551111,1553111511,5135151153,5351313331,

%U 15533353153,35553111153,35555511111,153335313531,155315331153,315331155153,355333351555,1353153151155

%N Triangular numbers composed of digits {1,3,5}.

%H Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr135.html">Tridigital Triangular Numbers</a>.

%F a(n) = A000217(A119114(n)). - _Michel Marcus_, Mar 22 2023

%t Select[Accumulate[Range[1000000]],Union[Drop[DigitCount[#], {1,5,2}]]=={0}&] (* _Harvey P. Dale_, Apr 16 2011 *)

%t Table[Select[FromDigits/@Tuples[{1,3,5},n],OddQ[Sqrt[8#+1]]&],{n,12}]//Flatten (* _Harvey P. Dale_, Aug 02 2022 *)

%Y Cf. A000217, A119114. See A119033 for a table of cross-references.

%K nonn,base

%O 1,2

%A _Giovanni Resta_, May 10 2006

%E a(25)-a(27) from _Tyler Busby_, Mar 22 2023