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 A036524 #12 Mar 22 2020 17:30:53 %S A036524 78,1770,72771,6787770,37779778,2476777771,7477727778,7077757275771, %T A036524 27778877747778,177775772774775,7777755777787716,172777077787777785, %U A036524 7987777777779277726,177577777275777787371,377679777777377717778,77773137747757777775775,777367777737777774977796,67775787670776777773777778 %N A036524 Smallest triangular number containing exactly n 7's. %t A036524 nsmall = Table[Infinity, 20]; %t A036524 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A036524 n0 = Count[IntegerDigits[p], 7]; %t A036524 If[nsmall[[n0]] > p, nsmall[[n0]] = p]]; %t A036524 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A036524 Cf. A048362, A036517-A036526. %K A036524 nonn,base %O A036524 1,1 %A A036524 _David W. Wilson_ %E A036524 a(13)-a(18) from _Giovanni Resta_, Oct 30 2019