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A059204
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Number of non-unimodal permutations of n items (i.e., those which do not simply go up for the first part and then down for the rest, but at some point go down then up).
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41
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0, 0, 0, 2, 16, 104, 688, 4976, 40192, 362624, 3628288, 39915776, 478999552, 6227016704, 87178283008, 1307674351616, 20922789855232, 355687428030464, 6402373705596928, 121645100408569856, 2432902008176115712, 51090942171708391424, 1124000727777605582848
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OFFSET
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0,4
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COMMENTS
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Number of permutations of [n] minus the number of compositions of n. - Zerinvary Lajos, Oct 16 2006
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LINKS
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FORMULA
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E.g.f.: (1+x)/(2*(1-x))-exp(2*x)/2.
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EXAMPLE
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a(3) = 2 since the possibilities are {BAC, CAB}. a(4) = 16 since the possibilites are {ACBD, ADBC, BACD, BADC, BCAD, BDAC, CABD, CADB, CBAD, CBDA, CDAB, DABC, DACB, DBAC, DBCA, DCAB}.
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MAPLE
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a:= n-> n!-ceil(2^(n-1)):
seq(a(n), n=0..30);
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MATHEMATICA
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nn=30; Range[0, nn]!CoefficientList[Series[1/(1-x)-Exp[2x]/2-1/2, {x, 0, nn}], x] (* Geoffrey Critzer, Mar 17 2014 *)
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PROG
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(PARI) x= 'x + O('x^50); concat([0, 0, 0], Vec(serlaplace((1+x)/(2*(1-x))-exp(2*x)/2))) \\ G. C. Greubel, Dec 28 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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