

A066052


Number of permutations in the symmetric group S_n with order >= 3.


3



0, 0, 2, 14, 94, 644, 4808, 39556, 360260, 3619304, 39881104, 478861448, 6226452296, 87175900720, 1307664018464, 20922743681264, 355687216296688, 6402372708414176, 121645095599130560, 2432901984417975904, 51090942051756747104, 1124000727158723041088, 25852016735627132757376
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OFFSET

1,3


COMMENTS

Permutations which contain a cycle with length >=3.
For elements of order >= 3 we can show that they can be represented as a product of two involutions. [Proof: Write down the reduced cycle representation of the element. Decompose each subcycle of order >=3 into a product of two involutions, and distribute/merge the 1 or 2 factors of the involutions. For subcycles of order o>=3 it suffices to show that the reduced cycle (1 2 3 4 ... o), equivalent to the 1line representation (2 3 4 .. o 1), can be written as a product of 2 involutions, because other cycles are mapped to such a product by a fixed permutation. A constructive proof here is: the reduced cycle (1 2 3 4 ... o) is the product of the two involutions (1 2)(3 o)(4 o1)(5 o2)... and (2 o)(3 o1)... The first involution swaps the first two elements and reverts the ordering of all others from the 3rd on, then the second involution reverts the order of the elements from the 2nd on. The net effect is a cyclic shift of the o elements.]  R. J. Mathar, Jan 04 2017
a(n) is the number of permutations of [n] that are not involutions (see first formula).  Joerg Arndt, Jul 06 2020


LINKS

Table of n, a(n) for n=1..23.


FORMULA

a(n) = n!  A000085(n).
E.g.f.: 1/(1x)  exp(x*(x+2)/2).
Conjecture: (n+3)*a(n) +(n^22*n1)*a(n1) +(n+1)*a(n2) (n1)*(n2)^2*a(n3)=0.  R. J. Mathar, Jan 04 2017
Conjecture: a(n) +(n3)*a(n1) +(2*n1)*a(n2) +n*(n2)*a(n3) 2*(n2)*(n3)*a(n4)=0.  R. J. Mathar, Jan 04 2017


MATHEMATICA

nn=20; Range[0, nn]! CoefficientList[Series[1/(1x)Exp[x+x^2/2], {x, 0, nn}], x] (* Geoffrey Critzer, Jan 09 2013 *)


CROSSREFS

Cf. A000085, A000142.
Sequence in context: A270063 A033169 A090410 * A122057 A164891 A141146
Adjacent sequences: A066049 A066050 A066051 * A066053 A066054 A066055


KEYWORD

nonn,easy


AUTHOR

Sharon Sela (sharonsela(AT)hotmail.com), Dec 29 2001


EXTENSIONS

More terms from Vladeta Jovovic, Dec 31 2001


STATUS

approved



