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 A060490 #8 Jan 30 2020 16:37:47
%S A060490 0,0,120,3030,24552,130740,551640,1997415,6470420,19219462,53187840,
%T A060490 138658760,343297780,812249250,1845669776,4044119530,8573706300,
%U A060490 17637474350,35294157340,68850086745,131179071560,244518601660,446576824800,800201972990,1408466719120
%N A060490 Number of 6-block ordered tricoverings of an unlabeled n-set.
%C A060490 A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering.
%H A060490 Andrew Howroyd, <a href="/A060490/b060490.txt">Table of n, a(n) for n = 1..1000</a>
%F A060490 a(n) = binomial(n + 19, 19) - 6*binomial(n + 9, 9) - 15*binomial(n + 7, 7) + 135*binomial(n + 3, 3) - 310*binomial(n + 1, 1) + 240*binomial(n, 0) - 45*binomial(n - 1, -1).
%F A060490 G.f.: -y^3*( -78600*y^3 + 271080*y^4 - 120 - 630*y + 13248*y^2 - 635805*y^5 + 4300*y^15 - 15840*y^14 + 32760*y^13 - 18240*y^12 - 114120*y^11 + 442800*y^10 - 915315*y^9 - 1371804*y^7 + 1305540*y^8 + 1081360*y^6 + 45*y^17 - 660*y^16)/(-1 + y)^20.
%F A060490 E.g.f. for ordered k-block tricoverings of an unlabeled n-set is exp( -x + x^2/2 + x^3/3*y/(1 - y))*Sum_{k>=0} 1/(1 - y)^binomial(k, 3)*exp( -x^2/2*1/(1 - y)^n)*x^k/k!.
%Y A060490 Column k=6 of A060492.
%Y A060490 Cf. A060094, A060484, A060491.
%K A060490 nonn
%O A060490 1,3
%A A060490 _Vladeta Jovovic_, Mar 20 2001
%E A060490 a(1)=a(2)=0 prepended and terms a(23) and beyond from _Andrew Howroyd_, Jan 30 2020