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A007483
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a(n) = 3*a(n-1) + 2*a(n-2), with a(0)=1, a(1)=5.
(Formerly M3875)
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18
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1, 5, 17, 61, 217, 773, 2753, 9805, 34921, 124373, 442961, 1577629, 5618809, 20011685, 71272673, 253841389, 904069513, 3219891317, 11467812977, 40843221565, 145465290649, 518082315077, 1845177526529, 6571697209741
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OFFSET
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0,2
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COMMENTS
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Number of subsequences of [1,...,2n+1] in which each odd number has an even neighbor. The even neighbor must differ from the odd number by exactly 1.
a(n) is the upper left term in the (n+1)-th matrix power of [(1,4); (1,2)] and is the INVERT transform of (1, 4, 4*2, 4*2^2, 4*2^3, 4*2^4, ...), i.e. of (1, 4, 8, 16, 32, 64, 128, ...).
The sequence is equal to row sums of an eigentriangle generated as follows: Let matrix A = an infinite lower triangle with (1, 4, 8, 16, ...) in every column and B = a triangle with (1, 1, 5, 17, 61, ...) as the rightmost diagonal and the rest zeros. Then the eigentriangle is A * B as follows: (1; 4, 1; 8, 4, 5; 16, 8, 20, 17; ...) with sums (1, 5, 17, 61, ...). Individual rows can be recovered by taking the dot product of (1, 4, 8, 16, ...) reversed and equal numbers of terms of(1, 1, 5, 17, ...). For example, 61 = (16, 8, 4, 1) dot (1, 1, 5, 17) = (16 + 8 + 20 + 17). (End)
The sequence is equal to A007482 convolved with (1, 2, 0, 0, 0, ...); i.e. (1 + 5x + 17x^2 + ...) = (1 + 3x + 11x^2 + 39x^3 + ...) * (1 + 2x). - Gary W. Adamson, Aug 08 2016
a(n) is the number of edge covers of the fan graph F(1,n+1) with a pendant attached to the vertex of degree n+1. - Feryal Alayont, Dec 08 2023
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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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G.f.: (1 + 2*x)/(1 - 3*x - 2*x^2).
a(n) = ((17 + 7*sqrt(17))/34)*((3 + sqrt(17))/2)^n* + ((17 - 7*sqrt(17))/34)*((3 - sqrt(17))/2)^n. - Paul Barry, Dec 08 2004
a(n) = upper left term in the 2 X 2 matrix [1,2; 2,2]^(n+1). Also [a(n), a(n+1)] = the 2 X 2 matrix [0,1; 2,3]^(n+1) * [1,1]. Example: [0,1; 2,3]^4 * [1,1] = [61, 217]. - Gary W. Adamson, Mar 16 2008
Also, for n>=2, a(n)=[1,2;2,2]^(n-1)*[1,2]*[1,2]. - John M. Campbell, Jul 09 2011
a(n) = (i*sqrt(2))^(n-1)*( i*sqrt(2)*ChebyshevU(n, -3*i/(2*sqrt(2))) + 2*ChebyshevU(n-1, -3*i/(2*sqrt(2))) ). - G. C. Greubel, Jun 28 2021
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EXAMPLE
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a(2) = 17 = (8, 4, 1) dot (1, 1, 5) = 8 + 4 + 5. - Gary W. Adamson, Aug 06 2016
a(2) counts the edge covers of F(1,3) with a pendant attached at the vertex of degree 3. This is the graph:
* -- *
/ | \
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*---*---*
An edge cover is a subset of the edges where each vertex is an endpoint of at least one edge. We show a one-to-one correspondence between subsequences of [1,...,5] and edge covers. Label edges connecting the top left vertex to the bottom vertices with odd numbers, 1, 3, 5, left to right. Label bottom edges with 2 (left) and 4 (right). An odd number in the subsequence means that edge is not in the edge cover. An even number means that edge is in. All bottom vertices are covered because if an odd edge is missing, an even edge covers the vertex attached to that odd. The pendant edge must be in every cover, so that edge covers both top vertices. (End)
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MATHEMATICA
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a[0]=1; a[1]=5; a[n_]:= a[n]= 3*a[n-1]+2*a[n-2]; Table[a[n], {n, 0, 23}] (* James C. McMahon, Dec 17 2023 *)
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PROG
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(Magma) [Floor((3/2+Sqrt(17)/2)^n*(1/2+7*Sqrt(17)/34)+(1/2-7*Sqrt(17)/34)*(3/2-Sqrt(17)/2)^n)+1: n in [0..30]]; // Vincenzo Librandi, Jul 09 2011
(Haskell)
a007483 n = a007483_list !! n
a007483_list = 1 : 5 : zipWith (+)
(map (* 3) $ tail a007483_list) (map (* 2) a007483_list)
(Sage)
@CachedFunction
def a(n): return 5^n if (n<2) else 3*a(n-1) + 2*a(n-2)
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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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