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A000211
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a(n) = a(n-1) + a(n-2) - 2, a(0) = 4, a(1) = 3.
(Formerly M2396 N0953)
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28
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4, 3, 5, 6, 9, 13, 20, 31, 49, 78, 125, 201, 324, 523, 845, 1366, 2209, 3573, 5780, 9351, 15129, 24478, 39605, 64081, 103684, 167763, 271445, 439206, 710649, 1149853, 1860500, 3010351, 4870849, 7881198
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OFFSET
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0,1
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COMMENTS
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Let I=I_n be the n X n identity matrix and P=P_n be the incidence matrix of the cycle (1,2,3,...,n). Then, for n>=3, a(n) is the number of (0,1) n X n matrices A<=P^(-1)+I+P with exactly two 1's in every row and column. - Vladimir Shevelev, Apr 11 2010
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REFERENCES
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J. Riordan, Discordant permutations, Scripta Math., 20 (1954), 14-23.
J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 233.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
R. P. Stanley, Enumerative Combinatorics I, Example 4.7.15, p. 252.
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LINKS
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FORMULA
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G.f.: 2/(1-x)+(2-x)/(1-x-x^2) = (4-5*x-x^2) / ((x-1)*(x^2+x-1)).
a(n) = Lucas number A000032(n) + 2.
Binomial transform of [4, -1, 3, -4, 7, -11, 18, ...], i.e., the series continues as a signed version of the Lucas series, A000204. - Gary W. Adamson, Nov 08 2007
a(n) = F(n-1) + F(n+1) + 2, where F(n) is the n-th Fibonacci number. - Zerinvary Lajos, Feb 01 2008; corrected by Michel Marcus, Jan 05 2021
E.g.f.: 2*exp(x/2)*(exp(x/2) + cosh(sqrt(5)*x/2)). - Ilya Gutkovskiy, Feb 01 2017
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MAPLE
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A000211:=-(1+z)*(4*z-3)/(z-1)/(z**2+z-1); # conjectured by Simon Plouffe in his 1992 dissertation; gives sequence except for the leading 4
with(combinat): seq(fibonacci(n-1)+fibonacci(n+1)+2, n=0..32); # Zerinvary Lajos, Feb 01 2008
a:= n-> (Matrix([[4, 1, 5]]). Matrix(3, (i, j)-> if (i=j-1) then 1 elif j=1 then [2, 0, -1][i] else 0 fi)^n)[1, 1]: seq(a(n), n=0..33); # Alois P. Heinz, Aug 01 2008
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MATHEMATICA
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Transpose[NestList[{Last[#], First[#]+Last[#]-2}&, {4, 3}, 40]] [[1]] (* Harvey P. Dale, Mar 22 2011 *)
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PROG
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(Haskell)
a000211 n = a000211_list !! n
a000211_list = 4 : 3 : map (subtract 2)
(zipWith (+) a000211_list (tail a000211_list))
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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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STATUS
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approved
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