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 A036521 #14 Mar 22 2020 23:42:02 %S A036521 45,4465,144453,544446,427444941,414446445,4440447441,444344944456, %T A036521 4404424444435,1046444944404441,14546344444444945,441441444448564440, %U A036521 4445444444946443841,444438244043444444046,4444844454444484014645,44641424644444444411441,4024414288444444444404441,44448461444441444448442420 %N A036521 Smallest triangular number containing exactly n 4's. %t A036521 nsmall = Table[Infinity, 20]; %t A036521 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A036521 n0 = Count[IntegerDigits[p], 4]; %t A036521 If[nsmall[[n0]] > p, nsmall[[n0]] = p]]; %t A036521 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A036521 Cf. A048359, A036517-A036526. %K A036521 nonn,base %O A036521 1,1 %A A036521 _David W. Wilson_ %E A036521 a(12)-a(18) from _Giovanni Resta_, Oct 30 2019