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A294697
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Number of permutations of [n] avoiding {1342, 2143, 3412}.
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1
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1, 1, 2, 6, 21, 73, 244, 790, 2505, 7839, 24320, 74998, 230243, 704359, 2148620, 6538740, 19859175, 60213343, 182304334, 551269876, 1665215691, 5025508101, 15154701002, 45668771716, 137542144181, 414029250493, 1245760375814, 3746896181970, 11265861084585, 33863485814629
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OFFSET
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0,3
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LINKS
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FORMULA
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O.g.f.: (1 - 2*x)*(1 - 6*x + 12*x^2 - 9*x^3 + 4*x^4)/((1 - x)^3*(1 - 3*x)*(1 - 3*x + x^2)).
a(n) = 9*a(n-1) - 31*a(n-2) + 52*a(n-3) - 45*a(n-4) + 19*a(n-5) - 3*a(n-6) for n>5. - Colin Barker, Nov 07 2017
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MAPLE
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p := (1-2*x)*(1-6*x+12*x^2-9*x^3+4*x^4) :
q := (1-x)^3*(1-3*x)*(1-3*x+x^2) :
taylor(p/q, x=0, 40) :
gfun[seriestolist](%) ;
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PROG
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(PARI) Vec((1 - 2*x)*(1 - 6*x + 12*x^2 - 9*x^3 + 4*x^4) / ((1 - x)^3*(1 - 3*x)*(1 - 3*x + x^2)) + O(x^40)) \\ Colin Barker, Nov 07 2017
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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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