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A007481
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Number of subsequences of [ 1,...,n ] in which each even number has an odd neighbor.
(Formerly M0828)
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5
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1, 2, 3, 7, 11, 25, 39, 89, 139, 317, 495, 1129, 1763, 4021, 6279, 14321, 22363, 51005, 79647, 181657, 283667, 646981, 1010295, 2304257, 3598219, 8206733, 12815247, 29228713, 45642179, 104099605, 162557031, 370756241, 578955451, 1320467933
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OFFSET
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0,2
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COMMENTS
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = 3*a(n-2) + 2*a(n-4).
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EXAMPLE
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For n=2, there are the following three subsequences of [1,2] with the desired property: empty, [1], [1,2].
For n=3, there are the following seven subsequences of [1,2,3] with the desired property: empty, [1], [3], [1,2], [2,3], [1,3], [1,2,3].
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MATHEMATICA
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LinearRecurrence[{0, 3, 0, 2}, {1, 2, 3, 7}, 40] (* Harvey P. Dale, Feb 29 2012 *)
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PROG
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(Haskell)
a007481 n = a007481_list !! n
a007481_list = 1 : 2 : 3 : 7 : zipWith (+)
(map (* 3) $ drop 2 a007481_list) (map (* 2) a007481_list)
(PARI) a(n)=([0, 1, 0, 0; 0, 0, 1, 0; 0, 0, 0, 1; 2, 0, 3, 0]^n*[1; 2; 3; 7])[1, 1] \\ Charles R Greathouse IV, Mar 02 2016
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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