login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A145220 a(n) is the number of even permutations (of an n-set) with exactly 2 fixed points. 3
1, 0, 0, 20, 45, 504, 3640, 33480, 333585, 3671360, 44053416, 572698620, 8017774765, 120266629560, 1924266062160, 32712523070864, 588825415257345, 11187682889912640, 223753657798223920, 4698826813762738020, 103374189902780192781, 2377606367763944486840 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,4

LINKS

Table of n, a(n) for n=2..23.

Bashir Ali and A. Umar, Some combinatorial properties of the alternating group, Southeast Asian Bulletin Math. 32 (2008), 823-830.

FORMULA

a(n) = (n(n-1)/2)*A003221(n-2), (n > 1).

E.g.f.: (x^2*(1-x^2/2) * exp(-x))/(2*(1-x)).

EXAMPLE

a(5) = 20 because there are exactly 20 even permutations (of a 5-set) having 2 fixed points, namely: (123), (132), (124), (142), (125), (152), (134), (143), (135), (153), (145), (154), (234), (243), (235), (253), (245), (254), (345), (354).

PROG

(PARI) x = 'x + O('x^30); Vec(serlaplace((x^2*(1-x^2/2) * exp(-x))/(2*(1-x)))) \\ Michel Marcus, Apr 04 2016

CROSSREFS

Cf. A003221, A145219, A145223.

Sequence in context: A044478 A228319 A236474 * A234266 A234259 A135286

Adjacent sequences:  A145217 A145218 A145219 * A145221 A145222 A145223

KEYWORD

nonn

AUTHOR

Abdullahi Umar, Oct 09 2008

EXTENSIONS

More terms from Alois P. Heinz, Nov 19 2013

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 15 16:13 EDT 2019. Contains 324142 sequences. (Running on oeis4.)