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A034182
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Number of not-necessarily-symmetric n X 2 crossword puzzle grids.
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20
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1, 5, 15, 39, 97, 237, 575, 1391, 3361, 8117, 19599, 47319, 114241, 275805, 665855, 1607519, 3880897, 9369317, 22619535, 54608391, 131836321, 318281037, 768398399, 1855077839, 4478554081, 10812186005, 26102926095, 63018038199, 152139002497, 367296043197
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OFFSET
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1,2
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COMMENTS
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n X 2 binary arrays with a path of adjacent 1's and no path of adjacent 0's from top row to bottom row. - R. H. Hardin, Mar 21 2002
Define a triangle with T(n,1) = T(n,n) = n*(n-1) + 1, n>=1, and its interior terms via T(r,c) = T(r-1,c) + T(r-1,c-1)+ T(r-2,c-1), 2<=c<r. This gives 1; 3,3; 7,7,7; 13,17,17,13; 21,37,41,37,21; etc. The row sums are 1, 6, 21, 60, 157, 394, etc., and the first differences of the row sums are this sequence. - J. M. Bergot, Mar 16 2013
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LINKS
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FORMULA
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a(n) = 2a(n-1) + a(n-2) + 4.
(1 + 5x + 15x^2 + ...) = (1 + 2x + 2x^2 + ...) * (1 + 3x + 7x^2 + ...), convolution of A040000 and left-shifted A001333.
a(n) = (-4 + (1-sqrt(2))^(1+n) + (1+sqrt(2))^(1+n))/2. G.f.: x*(1+x)^2/((1-x)*(1 - 2*x - x^2)). - Colin Barker, May 22 2012
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MATHEMATICA
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{1}~Join~NestList[{#2, 2 #2 + #1 + 4} & @@ # &, {1, 5}, 28][[All, -1]] (* Michael De Vlieger, Oct 02 2017 *)
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PROG
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(Haskell)
a034182 n = a034182_list !! (n-1)
a034182_list = 1 : 5 : (map (+ 4) $
zipWith (+) a034182_list (map (* 2) $ tail a034182_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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STATUS
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approved
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