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%I #11 Sep 08 2022 08:45:25
%S 1,5,10,18,58,101,149,1050,1885,5925,10114,15173,47226,550594,585005,
%T 1049925,1533085,56174285,104987349,123053789,1017425330,1230586669,
%U 1245114914,38878044589,101111538130,177627538130,4728038741701
%N Numbers k such that the k-th triangular number contains only digits {1,5,7}.
%H G. Resta, <a href="http://www.numbersaplenty.com/tr/tr157.html">Tridigital Triangular Numbers</a>
%t Select[Range[2 10^7], Complement[IntegerDigits[Binomial[# + 1, 2]], {1, 5, 7}] == {}&] (* _Vincenzo Librandi_, Oct 07 2015 *)
%o (Magma) [n: n in [1..2*10^7] | Set(Intseq(Binomial(n+1, 2))) subset [1, 5, 7]]; // _Vincenzo Librandi_, Oct 07 2015
%Y Cf. A000217, A119134. See A119034 for a table of cross-references.
%K nonn,base
%O 1,2
%A _Giovanni Resta_, May 10 2006