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A116767
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Number of permutations of length n which avoid the patterns 1234, 3142, 3421.
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0
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1, 2, 6, 21, 71, 208, 526, 1174, 2370, 4416, 7714, 12783, 20277, 31004, 45946, 66280, 93400, 128940, 174798, 233161, 306531, 397752, 510038, 647002, 812686, 1011592, 1248714, 1529571, 1860241, 2247396
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OFFSET
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1,2
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LINKS
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FORMULA
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G.f.: A(x) = -{x(2x^8-3x^7+x^6-9x^5+15x^4-14x^3+13x^2-5x+1)}/{(x-1)^7}
For n >= 3, a(n) = (n^6 + 45n^5 - 245n^4 - 465n^3 + 8164n^2 - 24780n + 25920)/720. - Franklin T. Adams-Watters, Sep 16 2006
a(1)=1, a(2)=2, a(3)=6, a(4)=21, a(5)=71, a(6)=208, a(7)=526, a(8)=1174, a(9)=2370, a(n)=7*a(n-1)-21*a(n-2)+35*a(n-3)-35*a(n-4)+21*a(n-5)- 7*a(n-6)+a (n-7) [From Harvey P. Dale, Aug 31 2011]
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MATHEMATICA
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Join[{1, 2, 6}, Table[(n^6+45n^5-245n^4-465n^3+8164n^2-24780n+25920)/ 720, {n, 4, 40}]] (* or *) Join[{1, 2}, LinearRecurrence[ {7, -21, 35, -35, 21, -7, 1}, {6, 21, 71, 208, 526, 1174, 2370}, 40]] (* Harvey P. Dale, Aug 31 2011 *)
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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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