OFFSET
0,3
REFERENCES
F. Harary and E. Palmer, Graphical Enumeration, Academic Press, 1973, page 88.
FORMULA
a(n) = A138107(n,n).
EXAMPLE
For n=2 we have two edges and two vertices, for a total of six graphs:
* one vertex with two loops attached to it
* one loop attached to each vertex
* two vertices linked by two edges in opposite directions
* two vertices linked by two edges in the same direction
* one vertex with a loop being pointed at
* one vertex with a loop being pointed away from.
CROSSREFS
KEYWORD
nonn
AUTHOR
Marko Riedel, Jun 09 2023
STATUS
approved