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 A263997 #15 Aug 17 2018 21:15:53 %S A263997 1,2,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15, %T A263997 15,16,16,17,17,18,18,19,19,20,20,21,21,22,22,23,23,24,24,25,25,26,26, %U A263997 27,27,28,28,29,29,30,30,31,31,32,32,33 %N A263997 Sequence of block lengths in a block spiral of width 1. %C A263997 A left-handed block spiral is created by the following pattern: Start with a 1 X 1 square [ a(1)=1 ]. Attach a block of length 2 and width 1 (a horizontal 1 X 2 rectangle) to the upper side of the square [ a(2)=2 ] pointing to the right. Attach to the corner a vertical 2 X 1 block (rectangle). This block has length 2, so a(3)=2. Continue with a horizontal 1 X 2 block, so a(4) = 2. And so on. See the sketch shown in the link. %H A263997 Hans G. Oberlack, <a href="/A263997/a263997.pdf">Sketch of a block spiral</a> %F A263997 a(n) = a(n-2) + 1 for n > 4. %F A263997 O.g.f.: x*(1+x-x^2-x^3+x^4)/((1-x)*(1-x^2)). From A008619. - _Wolfdieter Lang_, Jan 05 2016 %Y A263997 Essentially the same as A008619. %K A263997 nonn,easy %O A263997 1,2 %A A263997 _Hans G. Oberlack_, Oct 31 2015 %E A263997 Edited by _Wolfdieter Lang_, Jan 05 2016