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A245891 Number of labeled increasing unary-binary trees on n nodes whose breadth-first reading word avoids 213 and 321. 1

%I #8 Mar 18 2018 17:31:53

%S 1,1,3,7,20,55,157,448

%N Number of labeled increasing unary-binary trees on n nodes whose breadth-first reading word avoids 213 and 321.

%C The number of labeled increasing unary-binary trees with an associated permutation avoiding 213 and 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.)

%C 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.

%H Manda Riehl, <a href="/A245891/a245891.png">The 7 trees when n = 4.</a>

%e When n=4, a(n)=7. In the Links above we show the seven labeled increasing trees on four nodes whose permutation avoids 213 and 321.

%Y A126223 gives the number of binary trees instead of unary-binary trees. A033638 gives the number of permutations which avoid 213 and 321 that are breadth-first reading words on labeled increasing unary-binary trees.

%K nonn,more

%O 1,3

%A _Manda Riehl_, Aug 19 2014

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Last modified March 29 06:57 EDT 2024. Contains 371265 sequences. (Running on oeis4.)