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A356696
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a(n) = Fibonacci(2n-1) - 2^n + binomial(n,2) + 2.
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0
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2, 1, 1, 2, 5, 14, 42, 128, 384, 1123, 3204, 8955, 24629, 66913, 180127, 481568, 1280855, 3393644, 8965476, 23633702, 62197602, 163483201, 429300366, 1126514817, 2954438135, 7745187919, 20297902537, 53182073798, 139315427369, 364898425658, 955648284654
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OFFSET
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0,1
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COMMENTS
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If n>0, number of Catalan words of length n avoiding the pattern 1234. The unrestricted Catalan words of length n are enumerated by A000108(n-1).
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LINKS
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FORMULA
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G.f.: ( 2-15*x+43*x^2-59*x^3+39*x^4-9*x^5 ) / ( (2*x-1)*(x^2-3*x+1)*(x-1)^3 ).
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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