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A200069
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a(n) = 4*a(n-1) + 13*a(n-2) for n>2, a(1)=1, a(2)=4.
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3
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1, 4, 29, 168, 1049, 6380, 39157, 239568, 1467313, 8983636, 55009613, 336825720, 2062427849, 12628445756, 77325345061, 473471175072, 2899114186081, 17751582020260, 108694812500093, 665549816263752, 4075231827556217, 24953074921653644, 152790313444845397
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OFFSET
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1,2
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COMMENTS
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De Moivres formula : a(n)=(r^n-s^n)/(r-s), for r>s gives sequences with integers if r and s are conjugates. With r=2+sqrt(17) and s=2-sqrt(17), a(n+1)/a(n) converges to 2+sqrt(17).
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LINKS
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FORMULA
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a(n) = ((2+sqrt(17))^n-(2-sqrt(17))^n)/(2*sqrt(17)).
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EXAMPLE
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a(3) = 4*4+13*1 = 29.
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MATHEMATICA
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LinearRecurrence[{4, 13}, {1, 4}, 50]
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PROG
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(Haskell)
a200069 n = a200069_list !! (n-1)
a200069_list = 1 : 4 : zipWith (+)
(map (* 4) $ tail a200069_list) (map (* 13) a200069_list)
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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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