OFFSET
1,3
COMMENTS
The number of labeled increasing unary-binary trees with an associated permutation avoiding 321 in the classical sense. The tree's permutation is found by recording the labels in the order in which they appear in a breadth-first search. (Note that a breadth-first search reading word is equivalent to reading the tree labels left to right by levels, starting with the root.)
In some cases, the same breadth-first search reading permutation can be found on differently shaped trees. This sequence gives the number of trees, not the number of permutations.
LINKS
Manda Riehl, The 9 trees when n=4.
EXAMPLE
When n=4, a(n)=9. In the Links above we show the nine labeled increasing trees on four nodes whose permutation avoids 321.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Manda Riehl, Aug 19 2014
STATUS
approved