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 A140789 #8 Mar 29 2014 03:07:29 %S A140789 0,1,8,32,448,2240,64512,556032,33521664,553615360,68717379584, %T A140789 2233380896768,562949684985856,36310271727239168,18446744004990074880, %U A140789 2370406613402957905920,2417851639194073977323520,620178945462269582252179456,1267650600228193372699684241408 %N A140789 We consider how many ways there are of coloring a square grid, n X n, using just two colors, black & white say. If the resulting grid has TWO lines of symmetry, then the number of different grids is given by this sequence. None of these counted are the images of any of the others under a rotation. If one wishes to count these as different, then each of these numbers can be multiplied by 2. %F A140789 a(2m) = 2^(m^2 - 1)*(2^m + 1) - 2^(m*(m + 1)/2). %F A140789 a(2m+1)= 2^(m^2 + 2*m + 1) - 2^((m^2 + 3*m + 2)/2). %o A140789 (PARI) s=[0]; for(m=1, 15, s=concat(s, [2^(m^2-1)*(2^m+1)-2^(m*(m+1)/2), 2^(m^2+2*m+1)-2^((m^2+3*m+2)/2)])); s \\ _Colin Barker_, Mar 28 2014 %Y A140789 Cf. A140650. %K A140789 nonn %O A140789 1,3 %A A140789 _Anthony C Robin_, Jul 14 2008 %E A140789 More terms from _Colin Barker_, Mar 28 2014