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A372458
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Coefficient of x^n in the expansion of 1 / ( (1-x) * (1-x-x^2)^2 )^n.
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2
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1, 3, 25, 225, 2129, 20723, 205471, 2063890, 20931585, 213864939, 2198044805, 22699471171, 235354244255, 2448409104820, 25544033624414, 267158874185420, 2800191197529633, 29405702263792875, 309320021637262225, 3258658594126096867, 34376186445159365709
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = Sum_{k=0..floor(n/2)} binomial(2*n+k-1,k) * binomial(4*n-k-1,n-2*k).
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x * (1-x) * (1-x-x^2)^2 ). See A368965.
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PROG
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(PARI) a(n, s=2, t=2, u=1) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial((t+u+1)*n-(s-1)*k-1, n-s*k));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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