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 A015220 #37 Mar 07 2022 13:05:43
%S A015220 0,4,10,20,56,84,120,220,286,364,560,680,816,1140,1330,1540,2024,2300,
%T A015220 2600,3276,3654,4060,4960,5456,5984,7140,7770,8436,9880,10660,11480,
%U A015220 13244,14190,15180,17296,18424,19600,22100,23426,24804,27720,29260,30856,34220,35990
%N A015220 Even tetrahedral numbers.
%H A015220 <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 A015220 From _Ant King_, Oct 19 2012: (Start)
%F A015220 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 A015220 a(n) = 64 + 3*a(n-3) - 3*a(n-6) + a(n-9).
%F A015220 G.f.: 2*x*(2+3*x+5*x^2+12*x^3+5*x^4+3*x^5+2*x^6) / ((1-x)^4*(1+x+x^2)^3).
%F A015220 Sum_{n>=1} 1/a(n) = 3/2*(1-log(2)). (End)
%F A015220 From _Amiram Eldar_, Mar 07 2022: (Start)
%F A015220 a(n) = A000292(A004772(n+1)).
%F A015220 Sum_{n>=1} (-1)^(n+1)/a(n) = 3*log(2) - 15/2 + 9*sqrt(2)*log(sqrt(2)+1)/2. (End)
%t A015220 LinearRecurrence[{1,0,3,-3,0,-3,3,0,1,-1},{0,4,10,20,56,84,120,220,286,364},41] (* _Ant King_, Oct 19 2012 *)
%t A015220 Select[Table[(Times@@(n+{0,1,2}))/6,{n,0,60}],EvenQ] (* _Harvey P. Dale_, Jan 22 2013 *)
%Y A015220 Cf. A000292, A004772, A015219.
%K A015220 nonn,easy
%O A015220 0,2
%A A015220 _Mohammad K. Azarian_
%E A015220 More terms from _Erich Friedman_
%E A015220 a(0) prepended by _Amiram Eldar_, Mar 07 2022