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A098589
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a(n) = 3*a(n-1) + 2*a(n-3), with a(0)=1, a(1)=3.
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1
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1, 3, 9, 29, 93, 297, 949, 3033, 9693, 30977, 98997, 316377, 1011085, 3231249, 10326501, 33001673, 105467517, 337055553, 1077170005, 3442445049, 11001446253, 35158678769, 112360926405, 359085671721, 1147574372701
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OFFSET
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0,2
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LINKS
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FORMULA
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G.f.: 1/(1-3*x-2*x^3).
a(n) = Sum_{k=0..floor(n/2)} Sum_{i=0..n} Sum_{j=0..i} C(n-2*k,i)*C(i,j) *C(j, k)*2^k.
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MATHEMATICA
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CoefficientList[Series[1/(1-3*x-2*x^3), {x, 0, 50}], x] (* or *) LinearRecurrence[{3, 0, 2}, {1, 3, 9}, 50] (* G. C. Greubel, Feb 03 2018 *)
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PROG
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(PARI) x='x+O('x^30); Vec(1/(1-3*x-2*x^3)) \\ G. C. Greubel, Feb 03 2018
(Magma) I:=[1, 3, 9]; [n le 3 select I[n] else 3*Self(n-1) + 2*Self(n-3): n in [1..30]]; // G. C. Greubel, Feb 03 2018
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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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