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A272073
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Exponents of x in the numerator of cluster variables of rank 2 cluster algebras.
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1
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3, 6, 15, 24, 57, 90, 213, 336, 795, 1254, 2967, 4680, 11073, 17466, 41325, 65184, 154227, 243270, 575583, 907896, 2148105, 3388314, 8016837, 12645360, 29919243, 47193126, 111660135, 176127144, 416721297, 657315450, 1555225053, 2453134656, 5804178915
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OFFSET
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2,1
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COMMENTS
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These numbers are the exponents of x in the numerator of the cluster algebra A(b,c) of rank 2 with parameters b=2 and c=3. Recall that the algebra A(2,3) is a subalgebra of Q(x,y) generated by the following infinite family of variables:
x(n) = (1+x(n-1)^2)/x(n-2) if n is even,
x(n) = (1+x(n-1)^3)/x(n-2) if n is odd.
with x(0)=x and x(1)=y.
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LINKS
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FORMULA
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a(n) = 4*a(n-2)-a(n-4), with a(2)=3, a(3)=6, a(4)=15, a(5)=24.
G.f.: 3*x^2*(1+x)^2 / (1-4*x^2+x^4). - Colin Barker, Apr 22 2016
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PROG
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(PARI) Vec(3*x^2*(1+x)^2/(1-4*x^2+x^4) + O(x^50)) \\ Colin Barker, Apr 22 2016
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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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