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 A124280 Expansion of 1/(1-x-x^2+x^3-x^4). 2
 1, 1, 2, 2, 4, 5, 9, 12, 20, 28, 45, 65, 102, 150, 232, 345, 529, 792, 1208, 1816, 2761, 4161, 6314, 9530, 14444, 21821, 33049, 49956, 75628, 114356, 173077, 261761, 396110, 599150, 906576, 1371377, 2074913, 3138864 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Diagonal sums of number triangle A124279. LINKS Index entries for linear recurrences with constant coefficients, signature (1,1,-1,1). FORMULA a(n) = sum{k=0..floor(n/2), sum{j=0..n-2k, C(j,n-2k-j)C(k,n-2k-j)}}. 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 CROSSREFS Sequence in context: A089935 A038000 A204856 * A088518 A001224 A102526 Adjacent sequences:  A124277 A124278 A124279 * A124281 A124282 A124283 KEYWORD easy,nonn AUTHOR Paul Barry, Oct 24 2006 STATUS approved

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Last modified August 18 16:13 EDT 2018. Contains 313833 sequences. (Running on oeis4.)