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 A036519 #14 Mar 22 2020 23:41:44 %S A036519 21,1225,23220,212226,22121226,2202322528,21222223210,202222242820, %T A036519 22122952222920,229222922294226,2221228522222920,222322222025202925, %U A036519 222212284222222920,22222122841222222920,2092226022222222442221,22222222042522220020225,622222207421222222222253,22222212062122272422225226 %N A036519 Smallest triangular number containing exactly n 2's. %t A036519 nsmall = Table[Infinity, 20]; %t A036519 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A036519 n0 = Count[IntegerDigits[p], 2]; %t A036519 If[nsmall[[n0]] > p, nsmall[[n0]] = p]]; %t A036519 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A036519 Cf. A048357, A036517-A036526. %K A036519 nonn,base %O A036519 1,1 %A A036519 _David W. Wilson_ %E A036519 a(12)-a(18) from _Giovanni Resta_, Oct 30 2019