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 A218329 #14 Aug 01 2015 10:38:24
%S A218329 10,34,80,266,420,624,1210,1606,2080,3290,4040,4896,6954,8170,9520,
%T A218329 12650,14444,16400,20826,23310,25984,31930,35216,38720,46410,50610,
%U A218329 55056,64714,69940,75440,87290,93654,100320,114586,122200,130144,147050,156026,165360
%N A218329 Even 9-gonal (nonagonal) pyramidal numbers.
%H A218329 <a href="/index/Rec#order_10">Index entries for linear recurrences with constant coefficients</a>, signature (1, 0, 3, -3, 0, -3, 3, 0, 1, -1).
%F A218329 a(n) = a(n-1) + 3*a(n-3) - 3*a(n-4) - 3*a(n-6) + 3*a(n-7) + a(n-9) - a(n-10).
%F A218329 a(n) = 3*a(n-3) - 3*a(n-6) + a(n-9) + 448.
%F A218329 a(n) = phi(n)*(phi(n)+9)*(7*phi(n)-36)/4374, where phi(n) = 3 + 12*n - 3*cos(2*n*Pi/3) + sqrt(3)*sin(2*n*Pi/3).
%F A218329 G.f.: 2*x*(5+12*x+23*x^2+78*x^3+41*x^4+33*x^5+29*x^6+3*x^7)/((1-x)^4*(1+x+x^2)^3).
%e A218329 The sequence of 9-gonal (nonagonal) pyramidal numbers A007584 begins 1, 10, 34, 80, 155, 266, 420, 624, 885, 1210,.... As the third even term is 80, then a(3) = 80.
%t A218329 LinearRecurrence[{1,0,3,-3,0,-3,3,0,1,-1},{10,34,80,266,420,624,1210,1606,2080,3290},39]
%Y A218329 Cf. A007584, A218328.
%K A218329 nonn
%O A218329 1,1
%A A218329 _Ant King_, Oct 28 2012