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 A048356 #26 Mar 07 2025 07:46:24 %S A048356 1,18,58,458,1541,6018,47141,149406,471404,4756366,15563541,149075893, %T A048356 149004101,149071198,14907133333,47322533981,471406654834, %U A048356 1510636363333,4714049577658,17954448535733,471151591552466,1490649602094141,4519177339165085,15434448560354601,47140495990478741 %N A048356 a(n) is the index of the smallest triangular number containing exactly n 1's. %H A048356 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a> %t A048356 nsmall = Table[Infinity, 20]; %t A048356 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A048356 n0 = Count[IntegerDigits[p], 1]; %t A048356 If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; %t A048356 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A048356 Cf. A036518, A048346, A048355, A048357, A048358, A048359, A048360, A048361, A048362, A048363, A048364, A048531. %K A048356 nonn,base %O A048356 1,2 %A A048356 _Patrick De Geest_, Mar 15 1999 %E A048356 a(15)-a(16) from _Lars Blomberg_, May 13 2011 %E A048356 a(17)-a(18) from _Giovanni Resta_, Oct 30 2019 %E A048356 a(19)-a(25) from _Max Alekseyev_, Mar 07 2025