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 A028992 #24 Feb 16 2025 08:32:35 %S A028992 0,24,46,154,204,396,474,750,856,1216,1350,1794,1956,2484,2674,3286, %T A028992 3504,4200,4446,5226,5500,6364,6666,7614,7944,8976,9334,10450,10836, %U A028992 12036,12450,13734,14176,15544,16014,17466,17964,19500,20026,21646,22200,23904 %N A028992 Even 9-gonal (or enneagonal) numbers. %H A028992 Vincenzo Librandi, <a href="/A028992/b028992.txt">Table of n, a(n) for n = 0..10000</a> %H A028992 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/NonagonalNumber.html">Nonagonal Number</a> %H A028992 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,-2,-1,1). %F A028992 a(n) = (1/2)*(28*(n-1)^2 + 60*(n-1) + 33 + (14*(n-1)+15)*(-1)^(n-1)). %F A028992 a(n) = a(n-1)+2*a(n-2)-2*a(n-3)-a(n-4)+a(n-5) for n>4. - _Colin Barker_, May 30 2015 %F A028992 G.f.: -2*x*(3*x^3+30*x^2+11*x+12) / ((x-1)^3*(x+1)^2). - _Colin Barker_, May 30 2015 %o A028992 (Magma) [(1/2)*(28*(n-1)^2 + 60*(n-1) + 33 + (14*(n-1)+15)*(-1)^(n-1)): n in [0..40]]; // _Vincenzo Librandi_, Aug 19 2011 %o A028992 (PARI) concat(0, Vec(-2*x*(3*x^3+30*x^2+11*x+12)/((x-1)^3*(x+1)^2) + O(x^100))) \\ _Colin Barker_, May 30 2015 %Y A028992 Cf. A001106. %Y A028992 Cf. A014642, A028991, A028994. %K A028992 nonn,easy %O A028992 0,2 %A A028992 _Patrick De Geest_ %E A028992 0 inserted, offset and formula corrected by _Omar E. Pol_, Aug 19 2011