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 A123831 #9 Nov 29 2017 03:39:31 %S A123831 2,24,88,1079,5563,74597,412309,5633649 %N A123831 Number of ways to build a contiguous building with n LEGO blocks of size 2 X 4 on top of a fixed block of the same size so that the building is symmetric after a rotation by 180 degrees. %C A123831 The base block is not counted among the n and must be the only block in the bottom layer of the building. %Y A123831 Cf. A112389, A123762 - A123827, A123829 - A123849. %K A123831 nonn,more %O A123831 1,1 %A A123831 _Søren Eilers_, Oct 29 2006