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 A172221 #16 Feb 16 2025 08:33:11
%S A172221 1,20,84,200,403,720,1180,1808,2631,3676,4970,6540,8413,10616,13176,
%T A172221 16120,19475,23268,27526,32276,37545,43360,49748,56736,64351,72620,
%U A172221 81570,91228,101621,112776,124720,137480,151083,165556,180926,197220
%N A172221 Number of ways to place 3 nonattacking zebras on a 3 X n board.
%C A172221 Zebra is a (fairy chess) leaper [2,3].
%H A172221 Vincenzo Librandi, <a href="/A172221/b172221.txt">Table of n, a(n) for n = 1..1000</a>
%H A172221 V. Kotesovec, <a href="https://oeis.org/wiki/User:Vaclav_Kotesovec">Number of ways of placing non-attacking queens and kings on boards of various sizes</a>
%H A172221 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ZebraGraph.html.html">Zebra Graph</a>
%H A172221 Wikipedia, <a href="https://en.wikipedia.org/wiki/Zebra_(chess)">Zebra (chess)</a>
%F A172221 a(n) = (9*n^3 - 21*n^2 + 50*n - 48)/2, n>=6.
%F A172221 G.f.: x*(2*x^8-4*x^7+2*x^6-8*x^5+28*x^4-20*x^3+10*x^2+16*x+1)/(x-1)^4. - _Vaclav Kotesovec_, Mar 25 2010
%t A172221 CoefficientList[Series[(2 x^8 - 4 x^7 + 2 x^6 - 8 x^5 + 28 x^4 - 20 x^3 + 10 x^2 + 16 x + 1) / (x - 1)^4, {x, 0, 50}], x] (* _Vincenzo Librandi_, May 28 2013 *)
%Y A172221 Cf. A172138, A061989.
%K A172221 nonn,easy
%O A172221 1,2
%A A172221 _Vaclav Kotesovec_, Jan 29 2010
%E A172221 More terms from _Vincenzo Librandi_, May 28 2013