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A182166 The total number of components of size 2 over all simple labeled graphs on n nodes. 3
1, 3, 12, 80, 960, 21504, 917504, 75497472, 12079595520, 3779571220480, 2322168557862912, 2810246167479189504, 6714614842830276788224, 31734302764884015836037120, 297105609428491265975789813760, 5516815412193254355313652349796352 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

LINKS

Alois P. Heinz, Table of n, a(n) for n = 2..50

FORMULA

E.g.f.: x^2/2 * G(x) where G(x) is e.g.f. for A006125.

a(n) = C(n,2) * 2^C(n-2,2). - Alois P. Heinz, Apr 16 2012

MAPLE

a:= n-> binomial(n, 2) *2^binomial(n-2, 2):

seq (a(n), n=2..20);  # Alois P. Heinz, Apr 16 2012

MATHEMATICA

nn = 20; g = Sum[2^Binomial[n, 2] x^n/n!, {n, 0, nn}]; Drop[Range[0, nn]! CoefficientList[Series[x^2/2 g, {x, 0, nn}], x], 2]

CROSSREFS

Sequence in context: A275488 A304561 A303227 * A303047 A197445 A303033

Adjacent sequences:  A182163 A182164 A182165 * A182167 A182168 A182169

KEYWORD

nonn

AUTHOR

Geoffrey Critzer, Apr 15 2012

STATUS

approved

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Last modified October 16 08:45 EDT 2019. Contains 328056 sequences. (Running on oeis4.)