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A268306
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The number of even permutations p of 1,2,...,n such that -1<=p(i)-i<=2 for i=1,2,...,n
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1
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1, 1, 2, 4, 7, 12, 22, 41, 75, 137, 252, 464, 853, 1568, 2884, 5305, 9757, 17945, 33006, 60708, 111659, 205372, 377738, 694769, 1277879, 2350385, 4323032, 7951296, 14624713, 26899040, 49475048, 90998801, 167372889, 307846737, 566218426
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OFFSET
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1,3
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LINKS
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FORMULA
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G.f.: x*(1-x+x^2-x^3-x^5) / ((1-x)*(1+x^2)*(1-x-x^2-x^3)). - Colin Barker, Jan 31 2016
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EXAMPLE
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There exist two even permutations p of 1,2,3 such that -1<=p(i)-i<=2 for i=1,2,3: (123) and (312), therefore a(3)=2.
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MATHEMATICA
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CoefficientList[Series[(1 - x + x^2 - x^3 - x^5)/((1 - x) (1 + x^2) (1 - x - x^2 - x^3)), {x, 0, 34}], x] (* Michael De Vlieger, Feb 01 2016 *)
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PROG
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(Maxima) t1:0$ t2:0$ t3:1$ for i:1 thru 100 do (a:1/2+sin((2*i+1)*%pi/4)/sqrt(2),
t:t1+t2+t3, t1:t2, t2:t3, t3:t, e:(t+a)/2, print(e));
(PARI) Vec(x*(1-x+x^2-x^3-x^5)/((1-x)*(1+x^2)*(1-x-x^2-x^3)) + O(x^50)) \\ Colin Barker, Jan 31 2016
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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