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 A123809 #12 Sep 16 2018 09:56:25 %S A123809 1,4,7,34,85,461,1175,6871,18807,112406,318571,1936113,5653062, %T A123809 34848639,103771770,645701202,1955873614 %N A123809 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 7 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 A123809 The base block is not counted among the n and must be the only block in the bottom layer of the building. %Y A123809 Cf. A112389, A123762 - A123827, A123829 - A123849. %K A123809 nonn %O A123809 1,2 %A A123809 _Søren Eilers_, Oct 29 2006 %E A123809 a(11)-a(17) from _Søren Eilers_, Sep 16 2018