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

%I #10 Jun 13 2015 00:52:16

%S 1,1,2,2,4,5,9,12,20,28,45,65,102,150,232,345,529,792,1208,1816,2761,

%T 4161,6314,9530,14444,21821,33049,49956,75628,114356,173077,261761,

%U 396110,599150,906576,1371377,2074913,3138864

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

%C Diagonal sums of number triangle A124279.

%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1,1).

%F a(n) = sum{k=0..floor(n/2), sum{j=0..n-2k, C(j,n-2k-j)C(k,n-2k-j)}}.

%F a(n) = a(n-1)+a(n-2)-a(n-3)+a(n-4), a(0)=1, a(1)=1, a(2)=2, a(3)=2. - _Carl Najafi_, May 06 2014

%K easy,nonn

%O 0,3

%A _Paul Barry_, Oct 24 2006