[seqfan] A002955: rooted trimmed trees
Joerg Arndt
arndt at jjj.de
Mon Mar 2 17:55:42 CET 2015
I observe that this sequence (seems to) count
"Unordered rooted trees without "x x" in the level
sequence for the pre-order walk."
Here are those (8) trees for n = 6:
1: [ 0 1 2 3 4 5 ]
O--o--o--o--o--o
2: [ 0 1 2 3 4 3 ]
O--o--o--o--o
.--o
3: [ 0 1 2 3 4 2 ]
O--o--o--o--o
.--o
4: [ 0 1 2 3 4 1 ]
O--o--o--o--o
.--o
5: [ 0 1 2 3 2 3 ]
O--o--o--o
.--o--o
6: [ 0 1 2 3 2 1 ]
O--o--o--o
.--o
.--o
7: [ 0 1 2 3 1 2 ]
O--o--o--o
.--o--o
8: [ 0 1 2 1 2 1 ]
O--o--o
.--o--o
.--o
Now I realize that I do _not_ understand the comments.
My understanding of "limb" is
"the way (subtree) from a leaf towards the root until
the first branch point (which can be the root) is reached."
Example: The tree
[ 0 1 2 3 4 2 1 2 1 ]
O--o--o--o--o
.--o
.--o--o
.--o
has limbs of lengths 4, 1, 2, and 1.
Now from what I read in A002955 it should count
trees without limbs of length 2 (? or length >= 2 ?).
The trees I show above are(?) different, they avoid subtrees
*--o
.--o
Can somebody clarify?
Best regards, jj
P.S.: should "unordered" be added to the name?
P.P.S.: should "free" be added to the names of
sequences that are about free trees (like A000055)?
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