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A003483
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Number of square permutations of n elements.
(Formerly M2931)
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19
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1, 1, 1, 3, 12, 60, 270, 1890, 14280, 128520, 1096200, 12058200, 139043520, 1807565760, 22642139520, 339632092800, 5237183952000, 89032127184000, 1475427973219200, 28033131491164800, 543494606861606400, 11413386744093734400, 235075995738558374400, 5406747901986842611200, 126214560713084056012800
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OFFSET
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0,4
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COMMENTS
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Number of permutations p in S_n such that there exists q in S_n with q^2=p.
"A permutation P has a square root if and only if the numbers of cycles of P that have each even length are even numbers." [Theorem 4.8.1. on p.147 from the Wilf reference]. - Joerg Arndt, Sep 08 2014
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REFERENCES
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Edward A. Bender, Asymptotic methods in enumeration, SIAM Review 16 (1974), no. 4, 485-515.
J. Blum, Enumeration of the square permutations in S_n, J. Combin. Theory, A 17 (1974), 156-161.
N. Pouyanne, On the number of permutations admitting an m-th root, Electron. J. Combin., 9 (2002), #R3.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.11.
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LINKS
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N. J. A. Sloane and Alois P. Heinz, Table of n, a(n) for n = 0..250 (first 101 terms from N. J. A. Sloane)
Steven R. Finch, Errata and Addenda to Mathematical Constants, p. 36.
P. Flajolet et al., A hybrid of Darboux's method and singularity analysis in combinatorial asymptotics, arXiv:math.CO/0606370, p. 18, Proposition 2.
M. R. Pournaki, On the number of even permutations with roots, The Australasian Journal of Combinatorics, Volume 45, 2009, pp. 37-42.
H. S. Wilf, Generatingfunctionology, 2nd edn., Academic Press, NY, 1994, p. 148, Eq. 4.8.1.
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FORMULA
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E.g.f.: sqrt((1+x)/(1-x)) * prod(k>=1, cosh( x^(2*k)/(2*k) ) ). [Blum, corrected].
a(2n+1) = (2n+1)*a(2n).
Asymptotics: a(n) ~ n! * sqrt(2/(n*Pi)) * e^G, where e^G = prod_{k>=1} cosh(1/(2k)) ~ 1.22177951519253683396485298445636121278881... (see A246945). - corrected by Vaclav Kotesovec, Sep 13 2014
G = Sum_{j>=1} (-1)^(j+1) * Zeta(2*j)^2 * (1-1/2^(2*j)) / (j * Pi^(2*j)). - Vaclav Kotesovec, Sep 20 2014
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EXAMPLE
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a(3)=3: permutations with square roots are identity and two 3-cycles.
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MAPLE
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with(combinat):
b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,
add(`if`(irem(i, 2)=0 and irem(j, 2)=1, 0, (i-1)!^j*
multinomial(n, n-i*j, i$j)/j!*b(n-i*j, i-1)), j=0..n/i)))
end:
a:= n-> b(n$2):
seq(a(n), n=0..30); # Alois P. Heinz, Sep 08 2014
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MATHEMATICA
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max = 20; f[x_] := Sqrt[(1 + x)/(1 - x)]* Product[ Cosh[x^(2*k)/(2*k)], {k, 1, max}]; se = Series[ f[x], {x, 0, max}]; CoefficientList[ se, x]*Range[0, max]! (* Jean-François Alcover, Oct 05 2011, after g.f. *)
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PROG
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(PARI)
N=66; x='x+O('x^66);
Vec(serlaplace( sqrt((1+x)/(1-x))*prod(k=1, N, cosh(x^(2*k)/(2*k)))))
\\ Joerg Arndt, Sep 08 2014
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CROSSREFS
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Cf. A103619 (cube root), A103620 (fourth root), A215716 (fifth root), A215717 (sixth root), A215718 (seventh root).
Column k=2 of A247005.
Cf. A246945, A247621.
Sequence in context: A069944 A253171 A073996 * A128602 A092803 A181282
Adjacent sequences: A003480 A003481 A003482 * A003484 A003485 A003486
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KEYWORD
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nonn,easy,nice
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AUTHOR
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N. J. A. Sloane.
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EXTENSIONS
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More terms from Vladeta Jovovic, Mar 28 2001
Additional comments from Michael Somos, Jun 27, 2002
Minor edits by Vaclav Kotesovec, Sep 16 2014 and Sep 21 2014
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STATUS
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approved
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