This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A115939 #20 Jul 14 2021 10:02:32 %S A115939 46097,49796,50139,55151,55484,56520,58050,62495,62567,62900,66294, %T A115939 68805,69542,70766,72594,73737,73971,74168,74357,75555,76364,77805, %U A115939 78686,78848,84555,85959,86076,87263,87669,88406,89883,90287,90297 %N A115939 Numbers k such that the k-th triangular number contains each of the 10 decimal digits exactly once. %C A115939 There are 82 such numbers, the largest being 138959. %C A115939 A118668(a(n)) = 10. - _Reinhard Zumkeller_, Jul 11 2015 %H A115939 Zak Seidov, <a href="/A115939/b115939.txt">Table of n, a(n) for n = 1..82</a> (full sequence) %e A115939 T(75555) = 2854316790. %t A115939 Select[Range[141000], Sort[IntegerDigits[# (# + 1)/2]] == Range[0, 9] &] (* _Giovanni Resta_, Mar 19 2013 *) %t A115939 (Sqrt[8#+1]-1)/2&/@Select[FromDigits/@Select[Permutations[Range[0,9]], #[[1]]>0&],OddQ[Sqrt[8#+1]]&] (* _Harvey P. Dale_, Sep 19 2016 *) %Y A115939 Cf. A000217, A115940, A118668. %K A115939 nonn,base,fini,full %O A115939 1,1 %A A115939 _Giovanni Resta_, Feb 06 2006