%N Sequence of block lengths in a block spiral of width 1.
%C 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 Hans G. Oberlack, <a href="/A263997/a263997.pdf">Sketch of a block spiral</a>
%F a(n) = a(n-2) + 1 for n > 4.
%F 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 Essentially the same as A008619.
%A _Hans G. Oberlack_, Oct 31 2015
%E Edited by _Wolfdieter Lang_, Jan 05 2016