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A138513
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a(n) = 8*a(n-1) - 5*a(n-2).
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1
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1, 3, 19, 137, 1001, 7323, 53579, 392017, 2868241, 20985843, 153545539, 1123435097, 8219753081, 60140849163, 440028027899, 3219519977377, 23556019679521, 172350557549283, 1261024361996659, 9226442108226857
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OFFSET
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1,2
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COMMENTS
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Rightmost digit of each term forms a cycle with period 4: 1, 3, 9, 7, ... (repeat) ...
Limit_{n->oo} a(n)/a(n-1) = 4 + sqrt(11) = 7.31662479...
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LINKS
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FORMULA
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a(n) = 8*a(n-1) - 5*a(n-2), n> 2; given a(1) = 1, a(2) = 3.
a(n) = upper left term of the 2 X 2 matrix [1,2; 1,7]^n * [1,0].
O.g.f.: x*(1-5*x)/(1-8*x+5*x^2). - R. J. Mathar, Apr 12 2008
a(n) = ((3*sqrt(11)/22 + 1/2)*(4 - sqrt(11))^n + ((-3*sqrt(11)/22 + 1/2)* (4 + sqrt(11))^n. - Emeric Deutsch, Apr 02 2008
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EXAMPLE
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a(5) = 1001 = 8*a(4) - 5*a(3) = 8*137 - 5*19.
a(5) = 1001 = upper left term in [1,2; 1,7]^5.
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MAPLE
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a[1]:=1: a[2]:=3: for n from 3 to 25 do a[n]:=8*a[n-1]-5*a[n-2] end do: seq(a[n], n=1..20); # Emeric Deutsch, Apr 02 2008
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MATHEMATICA
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LinearRecurrence[{8, -5}, {1, 3}, 50] (* G. C. Greubel, Sep 28 2017 *)
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PROG
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(PARI) x='x+O('x^50); Vec(x*(1-5*x)/(1-8*x+5*x^2)) \\ G. C. Greubel, Sep 28 2017
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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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EXTENSIONS
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STATUS
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approved
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