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 A048359 #21 Mar 07 2025 07:46:38 %S A048359 9,94,537,1043,29238,28790,94238,942703,2967970,45748113,170565790, %T A048359 939618480,2981759361,29814031731,94285146809,298802358238, %U A048359 2837045748113,9428516473384,29810549959517,537482082424790,699205897721757,2974721655733337,29032426496744790,29814339990932029,1043498506508221519 %N A048359 a(n) is the index of the smallest triangular number containing exactly n 4's. %H A048359 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a> %t A048359 nsmall = Table[Infinity, 20]; %t A048359 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A048359 n0 = Count[IntegerDigits[p], 4]; %t A048359 If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; %t A048359 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A048359 Cf. A036521, A048349, A048355, A048356, A048357, A048358, A048360, A048361, A048362, A048363, A048364, A048537. %K A048359 nonn,base %O A048359 1,1 %A A048359 _Patrick De Geest_, Mar 15 1999 %E A048359 a(13)-a(14) from _Lars Blomberg_, May 13 2011 %E A048359 a(15)-a(18) from _Giovanni Resta_, Oct 30 2019 %E A048359 a(19)-a(25) from _Max Alekseyev_, Mar 07 2025