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A030435 Expansion of (1+x-2*x^2-x^3)/(1/2-2*x^2+x^4). 1

%I

%S 2,2,4,6,12,20,40,68,136,232,464,792,1584,2704,5408,9232,18464,31520,

%T 63040,107616,215232,367424,734848,1254464,2508928,4283008,8566016,

%U 14623104,29246208,49926400,99852800

%N Expansion of (1+x-2*x^2-x^3)/(1/2-2*x^2+x^4).

%C Also (starting 4,6,...) the number of zig-zag paths from top to bottom of a rectangle of width 7, whose color is that of the top right corner. [From _Joseph Myers_, Dec 24 2008]

%H Joseph Myers, <a href="http://www.polyomino.org.uk/publications/2008/bmo1-2009-q1.pdf">BMO 2008--2009 Round 1 Problem 1---Generalisation</a>

%o (PARI) Vec((1+x-2*x^2-x^3)/(1/2-2*x^2+x^4)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 27 2012

%Y a(n)=2 A030436(n).

%K nonn,easy

%O 0,1

%A _N. J. A. Sloane_.

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Last modified March 25 15:02 EDT 2019. Contains 321470 sequences. (Running on oeis4.)