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A152120
a(n) = 2^n * Product_{k=1..(n-1)/2} (2 + 3*cos(k*Pi/n)^2).
0
1, 2, 4, 22, 56, 290, 748, 3862, 9968, 51458, 132820, 685654, 1769768, 9136034, 23581372, 121733590, 314211296, 1622045954, 4186725796, 21613041046, 55786259480, 287984161058, 743327100556, 3837260885398, 9904503072464
OFFSET
0,2
FORMULA
From Colin Barker, Jan 05 2014: (Start)
Conjecture: a(n) = 14*a(n-2) - 9*a(n-4) for n > 4.
G.f.: (9*x^4-6*x^3-10*x^2+2*x+1) / (9*x^4-14*x^2+1). (End)
MATHEMATICA
a = Table[2^n*Product[2 + 3*Cos[k*Pi/n]^2, {k, 1, (n - 1)/2}], {n, 0, 30}]; Round[%] FullSimplify[ExpandAll[a]]
CROSSREFS
Sequence in context: A348785 A067654 A271944 * A072355 A134246 A254867
KEYWORD
nonn
AUTHOR
STATUS
approved