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 A048360 #21 Mar 07 2025 07:52:18 %S A048360 5,10,105,714,5010,34221,103494,317885,3243010,1054130,55708989, %T A048360 280555005,1051237885,5206833885,30184681729,105409283410, %U A048360 1053999578610,1005540238434,8894447217885,178132285431643,717642746162114,3273082806882696,10445626412576275,33332433321191885,105428230332824189 %N A048360 a(n) is the index of the smallest triangular number containing exactly n 5's. %H A048360 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a> %t A048360 nsmall = Table[Infinity, 20]; %t A048360 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A048360 n0 = Count[IntegerDigits[p], 5]; %t A048360 If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; %t A048360 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A048360 Cf. A036522, A048350, A048355, A048356, A048357, A048358, A048359, A048361, A048362, A048363, A048364, A048539. %K A048360 nonn,base %O A048360 1,1 %A A048360 _Patrick De Geest_, Mar 15 1999 %E A048360 a(13)-a(15) from _Lars Blomberg_, May 16 2011 %E A048360 a(16)-a(18) from _Giovanni Resta_, Oct 30 2019 %E A048360 a(19)-a(25) from _Max Alekseyev_, Mar 07 2025