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 A162674 #17 Nov 20 2021 11:34:36 %S A162674 0,1,97,956,4780,16745,46921,112672,241536,474585,870265,1508716, %T A162674 2496572,3972241,6111665,9134560,13311136,18969297,26502321,36377020, %U A162674 49142380,65438681,86007097,111699776,143490400,182485225,229934601 %N A162674 Number of different fixed (possibly) disconnected tetrominoes bounded (not necessarily tightly) by an n X n square. %C A162674 Fixed quasi-tetrominoes. %H A162674 <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (7,-21,35,-35,21,-7,1). %F A162674 a(n) = n*(n-1)*(8*n^4-16*n^3-9*n^2+17*n+8)/12. %F A162674 G.f.: x^2*(1+90*x+298*x^2+90*x^3+x^4)/(1-x)^7. [_Colin Barker_, Apr 25 2012] %e A162674 a(2)=1: the (connected) square tetromino. %Y A162674 Cf. A162673, A162675, A162676, A162677, A094171 (free quasi-tetrominoes). %K A162674 nonn,easy %O A162674 1,3 %A A162674 _David Bevan_, Jul 27 2009