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 A197487 #31 May 30 2024 03:06:44
%S A197487 1,2,50,12792,30844560,671869521960,129553882116606720
%N A197487 Number of nonsingular n X n matrices with elements from {0,1,2}.
%H A197487 Minfeng Wang, <a href="/A197487/a197487.cpp.txt">C++ program to calculate A197487</a>
%t A197487 (* 2x2 case *) cnt = 0; Do[d = Det[{{a, b}, {c, d}}]; If[d != 0, cnt++], {a, 0, 2}, {b, 0, 2}, {c, 0, 2}, {d, 0, 2}]; cnt (* _T. D. Noe_, Nov 29 2011 *)
%Y A197487 Cf. A055165, A056989.
%K A197487 nonn,more
%O A197487 0,2
%A A197487 _Roger Hui_, Nov 29 2011
%E A197487 a(5)-a(6) from _Minfeng Wang_, May 29 2024
%E A197487 a(0)=1 prepended by _Alois P. Heinz_, May 29 2024