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A005285
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Number of permutations of (1,...,n) having n-7 inversions (n>=7).
(Formerly M4414)
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5
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1, 7, 35, 155, 649, 2640, 10569, 41926, 165425, 650658, 2554607, 10020277, 39287173, 154022930, 603919164, 2368601685, 9293159292, 36476745510, 143239635450, 562744102479, 2211876507387, 8697839966552, 34218338900591
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OFFSET
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7,2
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COMMENTS
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Sequence is a diagonal of the triangle A008302 (number of permutations of (1,...,n) with k inversions; see Table 1 of the Margolius reference). - Emeric Deutsch, Aug 02 2014
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REFERENCES
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S. R. Finch, Mathematical Constants, Cambridge, 2003, Section 5.14., p.356.
R. K. Guy, personal communication.
E. Netto, Lehrbuch der Combinatorik. 2nd ed., Teubner, Leipzig, 1927, p. 96.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = 2^(2*n-8)/sqrt(Pi*n)*Q*(1+O(n^{-1})), where Q is a digital search tree constant, Q = 0.2887880951... (see A048651). - corrected by Vaclav Kotesovec, Mar 16 2014
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EXAMPLE
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a(8)=7 because we have 21345678, 13245678, 12435678, 12354678, 12346578, 12345768, and 12345687.
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MAPLE
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g := proc(n, k) option remember; if k=0 then return(1) else if (n=1 and k=1) then return(0) else if (k<0 or k>binomial(n, 2)) then return(0) else g(n-1, k)+g(n, k-1)-g(n-1, k-n) end if end if end if end proc; seq(g(j+7, j), j=0..30); # Barbara Haas Margolius, May 31 2001
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MATHEMATICA
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Table[SeriesCoefficient[Product[(1-x^j)/(1-x), {j, 1, n}], {x, 0, n-7}], {n, 7, 25}] (* Vaclav Kotesovec, Mar 16 2014 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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More terms, asymptotic formula from Barbara Haas Margolius (margolius(AT)math.csuohio.edu), May 31 2001
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STATUS
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approved
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