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A327227 Number of labeled simple graphs covering n vertices with at least one endpoint/leaf. 21
0, 0, 1, 3, 31, 515, 15381, 834491, 83016613, 15330074139, 5324658838645, 3522941267488973, 4489497643961740521, 11119309286377621015089, 53893949089393110881259181, 513788884660608277842596504415, 9669175277199248753133328740702449 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

Covering means there are no isolated vertices.

A leaf is an edge containing a vertex that does not belong to any other edge, while an endpoint is a vertex belonging to only one edge.

Also graphs with minimum vertex-degree 1.

LINKS

Table of n, a(n) for n=0..16.

FORMULA

Inverse binomial transform of A245797, if we assume A245797(0) = 0.

EXAMPLE

The a(4) = 31 edge-sets:

  {12,34}  {12,13,14}  {12,13,14,23}

  {13,24}  {12,13,24}  {12,13,14,24}

  {14,23}  {12,13,34}  {12,13,14,34}

           {12,14,23}  {12,13,23,24}

           {12,14,34}  {12,13,23,34}

           {12,23,24}  {12,14,23,24}

           {12,23,34}  {12,14,24,34}

           {12,24,34}  {12,23,24,34}

           {13,14,23}  {13,14,23,34}

           {13,14,24}  {13,14,24,34}

           {13,23,24}  {13,23,24,34}

           {13,23,34}  {14,23,24,34}

           {13,24,34}

           {14,23,24}

           {14,23,34}

           {14,24,34}

MATHEMATICA

Table[Length[Select[Subsets[Subsets[Range[n], {2}]], Union@@#==Range[n]&&Min@@Length/@Split[Sort[Join@@#]]==1&]], {n, 0, 5}]

CROSSREFS

The non-covering version is A245797.

The generalization to set-systems is A327229.

BII-numbers of set-systems with minimum degree 1 are A327105.

Cf. A001187, A006129, A059166, A059167, A100743, A136284, A327079, A327098, A327103, A327228, A327230.

Sequence in context: A212917 A223993 A143637 * A245109 A121563 A261471

Adjacent sequences:  A327224 A327225 A327226 * A327228 A327229 A327230

KEYWORD

nonn

AUTHOR

Gus Wiseman, Sep 01 2019

STATUS

approved

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Last modified February 23 06:13 EST 2020. Contains 332159 sequences. (Running on oeis4.)