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 A290768 #14 Jun 04 2025 00:52:47
%S A290768 3,6,12,21,33,48,66,87,111,138,168,201,237,276,318,363,411,462,516,
%T A290768 573,633,696,762,831,903,978,1056,1137,1221,1308,1398,1491,1587,1686,
%U A290768 1788,1893,2001,2112,2226,2343,2463,2586,2712,2841,2973,3108,3246,3387,3531,3678
%N A290768 a(n) = (3/2)*(n^2 - n + 2).
%C A290768 For n > 2, also the number of (non-null) connected induced subgraphs in the n-pan graph.
%H A290768 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ConnectedGraph.html">Connected Graph</a>.
%H A290768 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PanGraph.html">Pan Graph</a>.
%H A290768 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Vertex-InducedSubgraph.html">Vertex-Induced Subgraph</a>.
%H A290768 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A290768 a(n) = (3/2)*(n^2 - n + 2).
%F A290768 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 3.
%F A290768 G.f.: -3*x*(1 - x + x^2)/(-1 + x)^3.
%F A290768 a(n) = 3*A000124(n-1). - _R. J. Mathar_, May 07 2024
%F A290768 E.g.f.: -3 + 3*exp(x)*(1 + x^2/2). - _Elmo R. Oliveira_, May 31 2025
%t A290768 Table[3/2 (n^2 - n + 2), {n, 20}]
%t A290768 LinearRecurrence[{3, -3, 1}, {3, 6, 12}, 20]
%t A290768 CoefficientList[Series[-((3 (1 - x + x^2))/(-1 + x)^3), {x, 0, 20}], x]
%Y A290768 Cf. A000124.
%K A290768 nonn,easy
%O A290768 1,1
%A A290768 _Eric W. Weisstein_, Aug 10 2017