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 A048357 #22 Mar 07 2025 07:46:12 %S A048357 6,49,215,651,6651,66367,206020,635959,6651759,21411348,66651759, %T A048357 666816649,666651759,6666651759,64687340681,210818509825, %U A048357 1115546688777,6666665142651,21411315850840,66642647340651,494412868404984,2108185106684051,21083748539063076,209872546190692806,494534695369742950 %N A048357 a(n) is the index of the smallest triangular number containing exactly n 2's. %H A048357 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a> %t A048357 nsmall = Table[Infinity, 20]; %t A048357 For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; %t A048357 n0 = Count[IntegerDigits[p], 2]; %t A048357 If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; %t A048357 ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *) %Y A048357 Cf. A036519, A048347, A048355, A048356, A048358, A048359, A048360, A048361, A048362, A048363, A048364, A048533. %K A048357 nonn,base %O A048357 1,1 %A A048357 _Patrick De Geest_, Mar 15 1999 %E A048357 a(14)-a(15) from _Lars Blomberg_, May 13 2011 %E A048357 a(16)-a(18) from _Giovanni Resta_, Oct 30 2019 %E A048357 a(19)-a(25) from _Max Alekseyev_, Mar 07 2025