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 A277991 #21 Sep 08 2022 08:46:17 %S A277991 0,72,306,702,1260,1980,2862,3906,5112,6480,8010,9702,11556,13572, %T A277991 15750,18090,20592,23256,26082,29070,32220,35532,39006,42642,46440, %U A277991 50400,54522,58806,63252,67860,72630,77562,82656,87912,93330,98910 %N A277991 a(n) = 81*n^2 - 9*n. %C A277991 For n > 0, a(n) is the second Zagreb index of the polycyclic aromatic hydrocarbon PAH[n]. The second Zagreb index of a simple connected graph is the sum of the degree products d(i)d(j) over all edges ij of the graph. The pictorial definition of PAH[n] can be viewed in the Farahani reference. %C A277991 The M-polynomial of the polycyclic aromatic hydrocarbon PAH[n] is M(PAH[n], x, y) = 6*n*x*y^3 + 3*n*(3*n-1)*x^3*y^3. %H A277991 E. Deutsch and Sandi Klavzar, <a href="http://dx.doi.org/10.22052/ijmc.2015.10106">M-polynomial and degree-based topological indices</a>, Iranian J. Math. Chemistry, 6, No. 2, 2015, 93-102. %H A277991 M. R. Farahani, <a href="http://www.jchemacta.com/index.php/amc/article/view/99">Some connectivity indices of polycyclic aromatic hydrocarbons (PAHs)</a>, Advances in Materials and Corrosion, 1, 2013, 65-69. %H A277991 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1). %F A277991 G.f.: 18*x*(4 + 5x)/(1 - x)^3. %F A277991 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). - _Vincenzo Librandi_, Nov 13 2016 %p A277991 seq(81*n^2-9*n, n = 1..35); %o A277991 (Magma) [81*n^2-9*n: n in [0..35]]; // _Vincenzo Librandi_, Nov 13 2016 %o A277991 (PARI) a(n)=81*n^2-9*n \\ _Charles R Greathouse IV_, Jun 17 2017 %Y A277991 Cf. A277990. %K A277991 nonn,easy %O A277991 0,2 %A A277991 _Emeric Deutsch_, Nov 12 2016