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A113070
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Expansion of ((1+x)/(1-2x))^2.
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2
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1, 6, 21, 60, 156, 384, 912, 2112, 4800, 10752, 23808, 52224, 113664, 245760, 528384, 1130496, 2408448, 5111808, 10813440, 22806528, 47972352, 100663296, 210763776, 440401920, 918552576, 1912602624, 3976200192, 8254390272, 17112760320
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OFFSET
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0,2
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COMMENTS
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Binomial transform is A014915. In general, ((1+x)/(1-r*x))^2 expands to a(n) = ((r+1)*r^n*((r+1)*n + r - 1) + 0^n)/r^2, which is also a(n) = Sum_{k=0..n} C(n,k)*Sum_{j=0..k} (j+1)*(r+1)^j. This is the self-convolution of the coordination sequence for the infinite tree with valency r.
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LINKS
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FORMULA
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G.f.: (1+x)^2/(1-2x)^2;
a(n) = 3*2^n(3n+1)/4 + 0^n/4;
a(n) = Sum_{k=0..n} C(n, k)*Sum_{j=0..k} (j+1)*3^j.
a(n) = 4*a(n-1) - 4*a(n-2); a(0)=1, a(1)=6, a(2)=21. - Harvey P. Dale, May 20 2011
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MATHEMATICA
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Join[{1}, LinearRecurrence[{4, -4}, {6, 21}, 30]] (* or *) CoefficientList[ Series[((1+x)/(1-2x))^2, {x, 0, 30}], x] (* Harvey P. Dale, May 20 2011 *)
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PROG
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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