A266910 - OEIS

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A266910

Number of size 2 subsets of S_n that generate a transitive subgroup of S_n.

1, 12, 210, 5520, 206760, 10473120, 688821840, 57039171840, 5805880778880, 712594633766400, 103804864923513600, 17709509301413529600, 3498328696524626764800, 792308057159314683187200, 203965258080479292004608000, 59229266937652347633377280000, 19270409372174365076286590976000

(list; graph; refs; listen; history; text; internal format)

OFFSET

2,2

LINKS

Table of n, a(n) for n=2..18.

FORMULA

a(n) = (A122949(n) - (n - 1)!)/2.

EXAMPLE

a(3) = 12 because there are 15 = binomial(3!,2) size 2 subsets of S_3 and every such subset generates a transitive subgroup of S_3 except: {(),(12)}, {(),(13)}, {(),(23)}.

MATHEMATICA

nn = 20; a = Sum[n!^2 x^n/n!, {n, 0, nn}]; Drop[(Range[0, nn]! CoefficientList[Series[Log[a], {x, 0, nn}], x] - Table[(n - 1)!, {n, 0, nn}])/2, 2]

CROSSREFS

Cf. A122949.

Sequence in context: A342502 A334886 A027399 * A296681 A231260 A317199

Adjacent sequences: A266907 A266908 A266909 * A266911 A266912 A266913

KEYWORD

nonn

AUTHOR

Geoffrey Critzer, Jan 05 2016

STATUS

approved