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 A048364 #21 Mar 07 2025 07:54:29 %S A048364 13,44,631,1413,28282,76419,282842,1414213,4471955,31616419,446313656, %T A048364 1414001413,6276925986,44710913656,44721356419,632129732586, %U A048364 527257052109,14141923468538,44698881216419,199449241362306,141421356237309,4690415742340969,42426404749855955,44703449755024186,1095435073375871053 %N A048364 a(n) is the index of the smallest triangular number containing exactly n 9's. %H A048364 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a> %t A048364 nsmall = Table[Infinity, 20]; %t A048364 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A048364 n0 = Count[IntegerDigits[p], 9]; %t A048364 If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; %t A048364 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A048364 Cf. A036526, A048354, A048355, A048356, A048357, A048358, A048359, A048360, A048361, A048362, A048363, A048547. %K A048364 nonn,base %O A048364 1,1 %A A048364 _Patrick De Geest_, Mar 15 1999 %E A048364 a(13)-a(15) from _Lars Blomberg_, May 16 2011 %E A048364 a(16)-a(18) from _Giovanni Resta_, Oct 30 2019 %E A048364 a(19)-a(25) from _Max Alekseyev_, Mar 07 2025