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A108489
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Expansion of 1/sqrt(1-2x-5x^2-6x^3+9x^4).
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1
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1, 1, 4, 13, 37, 130, 427, 1441, 4954, 16987, 58843, 204610, 713893, 2500183, 8778478, 30898309, 108987427, 385136680, 1363252603, 4832572951, 17153677534, 60961916965, 216887253409, 772400234074, 2753261490919, 9822393082513
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OFFSET
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0,3
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COMMENTS
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In general, Sum_{k=0..n}, C(n-k,k)^2*a^k*b^(n-k) has expansion 1/sqrt(1-2bx-(2ab-b^2)x^2-2a*b^2*x^3+(ab)^2*x^4).
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LINKS
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FORMULA
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a(n) = Sum_{k=0..n}, C(n-k, k)^2*3^k.
D-finite with recurrence: n*a(n) +(-2*n+1)*a(n-1) +5*(-n+1)*a(n-2) +3*(-2*n+3)*a(n-3) +9*(n-2)*a(n-4)=0. - R. J. Mathar, Feb 20 2015
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MATHEMATICA
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Array[Sum[Binomial[# - k, k]^2*3^k, {k, 0, #}] &, 26, 0] (* Michael De Vlieger, Sep 10 2021 *)
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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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