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 A036520 #14 Mar 22 2020 23:41:55 %S A036520 3,3003,33153,3303735,333336,333039336,13333363350,330335331336, %T A036520 3033333513330,303313733373336,30333330330333465,313631353332333336, %U A036520 330332337333331336,3333043333303433331,3333333330453337336653,30733333373333335323321,330332334339335333331336,33033333933433336339893330 %N A036520 Smallest triangular number containing exactly n 3's. %t A036520 nsmall = Table[Infinity, 20]; %t A036520 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A036520 n0 = Count[IntegerDigits[p], 3]; %t A036520 If[nsmall[[n0]] > p, nsmall[[n0]] = p]]; %t A036520 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A036520 Cf. A048358, A036517-A036526. %K A036520 nonn,base %O A036520 1,1 %A A036520 _David W. Wilson_ %E A036520 a(12)-a(18) from _Giovanni Resta_, Oct 30 2019