2,4

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

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

Egf.: (x^2(1-x^2/2)e^(-x))/2(1-x)

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).

A003221

Sequence in context: A134619 A044097 A044478 this_sequence A135286 A053245 A115882

Adjacent sequences: A145217 A145218 A145219 this_sequence A145221 A145222 A145223

nonn

A. Umar (aumarh(AT)squ.edu.om), Oct 09 2008