OFFSET
1,3
LINKS
N. J. A. Sloane and Thomas Wieder, The Number of Hierarchical Orderings, Order 21 (2004), 83-89.
EXAMPLE
Let : denote the separation between two subhierarchies, e.g. 2:3 are two subhierarchies where subhierarchy s=1 contains two elements and subhierarchy s=2 contains three elements. Let | denote the separation between two levels, e.g. 2|2|1 is a hierarchy composed of three levels with two elements on levels l=1 and l=2 and one element on level l=3. For n=5 one has a(5) = 17 hierarchies where at least one subhierarchy has exactly 2 levels (and no level l > 2 is allowed):
4|1; 1|4; 3|2; 2|3; 2|2:1; 2|1:2; 1|2:2; 2|1:1|1; 1|2:1|1; 2|1:1:1; 1|2:1:1;
1|1:1:1:1; 1|3:1; 3|1:1; 1|1:2:1; 1|1:1|1:1; 1|1:3.
CROSSREFS
KEYWORD
nonn
AUTHOR
Thomas Wieder, Aug 13 2004
STATUS
approved