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A152119
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a(n) = Product_{k=1..(n-1)/2} (5 + 4*cos(k*Pi/n)^2).
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1
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1, 1, 1, 6, 7, 41, 48, 281, 329, 1926, 2255, 13201, 15456, 90481, 105937, 620166, 726103, 4250681, 4976784, 29134601, 34111385, 199691526, 233802911, 1368706081, 1602508992, 9381251041, 10983760033, 64300051206, 75283811239
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OFFSET
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0,4
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LINKS
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FORMULA
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a(n) = Product_{k=1..(n-1)/2} (5 + 4*cos(k*Pi/n)^2).
a(n) = 7*a(n-2) - a(n-4).
G.f.: (x^4 - x^3 - 6*x^2 + x + 1)/((x^2 - 3*x + 1)*(x^2 + 3*x + 1)). (End)
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MATHEMATICA
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a = Table[Product[5 + 4*Cos[k*Pi/n]^2, {k, 1, (n - 1)/2}], {n, 0, 10}]; FullSimplify[ExpandAll[a]]
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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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