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A164651
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Number of permutations of length n that avoid both 1243 and 2134.
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1
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1, 1, 2, 6, 22, 87, 354, 1459, 6056, 25252, 105632, 442916, 1860498, 7826120, 32956964, 138911074, 585926818, 2472923499, 10442263142, 44112331275, 186413949540, 788000866243, 3331853294090, 14090947775581, 59604161832772
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OFFSET
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0,3
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COMMENTS
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Le proved that this also gives the number of permutations of length n that avoid both 1342 and 3124.
For n>=1, a(n) is the number of paths of North steps N = (0,1), East steps E = (1,0), and Diagonal steps D = (1,1) from the origin to (n-1,n-1) such that all D steps lie on the diagonal line y = x and the first step away from the diagonal (if there is one) is a North step. For example, a(3) = 6 counts DD, DNE, NED, NENE, NEEN, NNEE. - David Callan, Jun 25 2013
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LINKS
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FORMULA
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G.f.: (3*x^2-9*x+2+x*(1-x)*sqrt(1-4*x))/(2*(x-1)*(x^2+4*x-1)).
Recurrence: (n-4)*(n-1)*a(n) = (9*n^2 - 51*n + 62)*a(n-1) - (23*n^2 - 145*n + 222)*a(n-2) + (11*n^2 - 73*n + 122)*a(n-3) + 2*(n-3)*(2*n-7)*a(n-4).
a(n) ~ (1/2-1/sqrt(5))*(sqrt(5)+2)^n. (End)
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MATHEMATICA
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CoefficientList[Series[(3*x^2-9*x+2+x*(1-x)*Sqrt[1-4*x])/(2*(x-1)*(x^2+4*x-1)), {x, 0, 20}], x] (* Vaclav Kotesovec, Oct 28 2012 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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