OFFSET
0,3
COMMENTS
Figure n3 demonstrates all 4*13=52 hierarchical orderings on n=3 connected points. In addition, the pink pictures describe the 10 cases where not all or no points are connected.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..76
Thomas Wieder, Figure n3
EXAMPLE
There is A001187(2)=1 connected graph for n=2 labeled elements: The chain 1-2.
The chain gives us 3 hierarchical orderings:
1-2
1
|
2
2
|
1
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Thomas Wieder, Jan 19 2008; corrected Jan 19 2008
EXTENSIONS
Offset corrected by Alois P. Heinz, Dec 16 2014
STATUS
approved