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 A162673 #17 Jul 22 2025 06:56:05
%S A162673 0,4,48,204,580,1320,2604,4648,7704,12060,18040,26004,36348,49504,
%T A162673 65940,86160,110704,140148,175104,216220,264180,319704,383548,456504,
%U A162673 539400,633100,738504,856548,988204,1134480,1296420,1475104,1671648,1887204
%N A162673 Number of different fixed (possibly) disconnected trominoes bounded (not necessarily tightly) by an n*n square.
%C A162673 Fixed quasi-trominoes.
%H A162673 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (5,-10,10,-5,1).
%F A162673 a(n)=n*(n-1)*(3*n^2-3*n-2)/2.
%F A162673 G.f.: 4*x^2*(1+7*x+x^2)/(1-x)^5. [_Colin Barker_, Apr 25 2012]
%e A162673 a(2)=4: the four rotations of the (connected) L tromino
%t A162673 LinearRecurrence[{5,-10,10,-5,1},{0,4,48,204,580},40] (* _Harvey P. Dale_, Aug 09 2017 *)
%Y A162673 Cf. A162674, A162676, A162677, A094170 (free quasi-trominoes).
%K A162673 nonn,easy
%O A162673 1,2
%A A162673 _David Bevan_, Jul 27 2009