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 A218325 #6 Aug 01 2015 10:37:19
%S A218325 8,26,60,196,308,456,880,1166,1508,2380,2920,3536,5016,5890,6860,9108,
%T A218325 10396,11800,14976,16758,18676,22940,25296,27808,33320,36330,39516,
%U A218325 46436,50180,54120,62608,67166,71940,82156,87608,93296,105400,111826,118508,132660
%N A218325 Even heptagonal pyramidal numbers.
%H A218325 <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 A218325 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 A218325 a(n) = 3*a(n-3) - 3*a(n-6) + a(n-9) + 320.
%F A218325 a(n) = (phi(n)+3)*(phi(n)+12)(5*phi(n)-3)/4374, where phi(n) = 12*n - 3*cos(2*n*pi/3) + sqrt(3)*sin(2*n*pi/3).
%F A218325 G. f. 2*x*(4+9*x+17*x^2+56*x^3+29*x^4+23*x^5+20*x^6+2*x^7) / ((1-x)^4*(1+x+x^2)^3).
%e A218325 The sequence of heptagonal pyramidal numbers A002413(n) begins 1, 8, 26, 60, 115, 196, 308, 456, 645, 880, … As the third even term is 60, then a(3) = 60.
%t A218325 LinearRecurrence[{1,0,3,-3,0,-3,3,0,1,-1},{8,26,60,196,308,456,880,1166,1508,2380},40]
%Y A218325 Cf. A002413, A218324.
%K A218325 nonn
%O A218325 1,1
%A A218325 _Ant King_, Oct 26 2012