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A001540
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Number of transpositions needed to generate permutations of length n.
(Formerly M1856 N0734)
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2
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0, 2, 8, 36, 184, 1110, 7776, 62216, 559952, 5599530, 61594840, 739138092, 9608795208, 134523132926, 2017846993904, 32285551902480, 548854382342176, 9879378882159186, 187708198761024552, 3754163975220491060, 78837443479630312280, 1734423756551866870182
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OFFSET
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1,2
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REFERENCES
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N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
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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E.g.f.: cosh(x)/(1-x) - exp(x).
Recurrence: a(n) = n*a(n-1) + n - (n mod 2).
a(n) = -1 + n!*Sum{k=0..floor(n/2)} 1/(2*k)! = -1 + round(n! * cosh(1)).
a(n) ~ [cosh(1)*n!] - 1, where [x] is the floor of x. - Simon Plouffe, Nov 28 2018
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EXAMPLE
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a(5)=-1+5!(1+1/2!+1/4!)=-1+120+60+5=184.
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MAPLE
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a := n -> (exp(1)*GAMMA(1 + n, 1) + exp(-1)*GAMMA(1 + n, -1))/2 - 1:
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MATHEMATICA
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With[{nn=20}, Rest[CoefficientList[Series[Cosh[x]/(1-x)-Exp[x], {x, 0, nn}], x]Range[0, nn]!]] (* Harvey P. Dale, Mar 04 2013 *)
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PROG
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(PARI) a(n)=-1+n!*sum(k=0, floor(n/2), 1/(2*k)!)
(Magma) [-1 + (&+[Factorial(n)/Factorial(2*k): k in [0..Floor(n/2)]]): n in [1..20]]; // G. C. Greubel, Nov 28 2018
(Sage) [-1 + factorial(n)*sum(1/factorial(2*k) for k in range(floor((n+2)/2))) for n in (1..20)] # G. C. Greubel, Nov 28 2018
(GAP) a:=[0];; for n in [2..20] do a[n]:=n*a[n-1]+n-(n mod 2); od; a; # Muniru A Asiru, Dec 05 2018
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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