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 A123793 #12 Sep 16 2018 09:56:33 %S A123793 1,3,5,18,42,170,405,1769,4507,20081,53008,240428,652160,3002052, %T A123793 8299727,38570944,108387118,507729121,1444491686,6809942459 %N A123793 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 5 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 A123793 The base block is not counted among the n and must be the only block in the bottom layer of the building. %Y A123793 Cf. A112389, A123762 - A123827, A123829 - A123849. %K A123793 nonn %O A123793 1,2 %A A123793 _Søren Eilers_, Oct 29 2006 %E A123793 a(12)-a(20) from _Søren Eilers_, Sep 16 2018