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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A211774 Number of rooted 2-regular labeled graphs on n nodes. 1
0, 0, 0, 3, 12, 60, 420, 3255, 28056, 270144, 2868840, 33293205, 419329020, 5697423732, 83069039508, 1293734268645, 21436030749840, 376516868504160, 6988441065717744, 136675039085498691, 2809247116432575420, 60543293881318183740, 1365186080156105513460 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..170

FORMULA

a(n) = n*A001205(n).

E.g.f.: x*A'(x) where A(x) = exp(-x/2-x^2/4)/sqrt(1-x) is the e.g.f. for A001205.

a(n) ~ sqrt(2) * n^(n+1) / exp(n+3/4). - Vaclav Kotesovec, Aug 22 2014

MAPLE

egf:= x *diff(exp(-x/2-x^2/4)/sqrt(1-x), x):

a:= n-> n! * coeff(series(egf, x, n+1), x, n):

seq(a(n), n=0..30);  # Alois P. Heinz, May 18 2012

MATHEMATICA

nn = 20; a = Log[1/(1 - x)]/2 - x/2 - x^2/4; Drop[Range[0, nn]! CoefficientList[Series[x D[Exp[a], x], {x, 0, nn}], x], 3]

CROSSREFS

Sequence in context: A159867 A082278 A078162 * A002497 A228251 A218092

Adjacent sequences:  A211771 A211772 A211773 * A211775 A211776 A211777

KEYWORD

nonn

AUTHOR

Geoffrey Critzer, May 18 2012

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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified April 26 03:51 EDT 2018. Contains 303098 sequences. (Running on oeis4.)