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 A123777 #11 Sep 16 2018 09:56:41 %S A123777 1,2,3,7,14,37,76,217,473,1374,3104,9196,21240,63929,150294,456703, %T A123777 1089683,3338624,8055267,24845592,60511113,187665038,460638240 %N A123777 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 3 on top of a fixed block of the same size so that the building is flat, i.e., with all blocks in parallel position and symmetric after a rotation by 180 degrees. %C A123777 The base block is not counted among the n and must be the only block in the bottom layer of the building. %Y A123777 Cf. A112389, A123762 - A123827, A123829 - A123849. %K A123777 nonn %O A123777 1,2 %A A123777 _Søren Eilers_, Oct 29 2006 %E A123777 a(16)-a(23) from _Søren Eilers_, Sep 16 2018