A145220 - OEIS

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A145220

a(n) is the number of even permutations (of an n-set) with exactly 2 fixed points.

1, 0, 0, 20, 45, 504, 3640, 33480, 333585, 3671360, 44053416, 572698620, 8017774765 (list; graph; refs; listen; history; internal format)

	OFFSET	`2,4`
	REFERENCES	`Ali, Bashir and Umar, A., "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)` `Egf.: (x^2(1-x^2/2)e^(-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).`
	CROSSREFS	`A003221` `Sequence in context: A134619 A044097 A044478 * A135286 A177725 A053245` `Adjacent sequences: A145217 A145218 A145219 * A145221 A145222 A145223`
	KEYWORD	`nonn`
	AUTHOR	`A. Umar (aumarh(AT)squ.edu.om), Oct 09 2008`

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Last modified February 17 10:57 EST 2012. Contains 206009 sequences.