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A069241
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Number of Hamiltonian paths in the graph on n vertices {1,...,n}, with i adjacent to j iff |i-j|<=2.
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3
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1, 1, 1, 3, 6, 10, 17, 28, 44, 68, 104, 157, 235, 350, 519, 767, 1131, 1665, 2448, 3596, 5279, 7746, 11362, 16662, 24430, 35815, 52501, 76956, 112797, 165325, 242309, 355135, 520490, 762830, 1117997, 1638520, 2401384, 3519416, 5157972
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,4
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COMMENTS
| Equivalently, the number of bandwidth-at-most-2 arrangements of a straight line of n vertices.
a(n) = A003274(n)/2, n>1.
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FORMULA
| a(n)=3s(n)+s(n-1)+s(n-2)-2-n, where s(n) = A000930(n). G.f.: (3+x+x^2)/(1-x-x^3)-(2-x)/(1-x)^2.
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EXAMPLE
| For example, the six Hamiltonian paths when n=4 are 1234, 1243, 1324, 1342, 2134, 3124.
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MAPLE
| a:= n-> (Matrix([[1, 1, 1, 0, 1]]). Matrix(5, (i, j)-> if i=j-1 then 1 elif j=1 then [3, -3, 2, -2, 1][i] else 0 fi)^n)[1, 3]: seq (a(n), n=0..38); [From Alois P. Heinz (heinz(AT)hs-heilbronn.de), Sep 09 2008]
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CROSSREFS
| Cf. A000930.
Sequence in context: A094272 A005045 A189376 * A092263 A170803 A182908
Adjacent sequences: A069238 A069239 A069240 * A069242 A069243 A069244
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KEYWORD
| nonn,easy
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AUTHOR
| D. E. Knuth, Apr 13 2002
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