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A245898 Number of permutations avoiding 231 that can be realized on increasing unary-binary trees with n nodes. 10
1, 1, 2, 4, 10, 26, 74, 217 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

The number of permutations avoiding 231 in the classical sense which can be realized as labels on an increasing unary-binary tree with n nodes read in the order they appear in a breadth-first search. (Note that breadth-first search reading word is equivalent to reading the tree left to right by levels, starting with the root.)

In some cases, more than one tree results in the same breadth-first search reading word, but here we count the number of permutations, not the number of trees.

LINKS

Table of n, a(n) for n=1..8.

EXAMPLE

For example, when n=4, the permutations 1234, 1243, 1324, and 1423 all avoid 231 in the classical sense and occur as breadth-first search reading words on an increasing unary-binary tree with 4 nodes:

       1           1            1              1

      / \         / \          / \            / \

     2   3       2   4        3   2          4   2

     |           |            |                  |

     4           3            4                  3

CROSSREFS

A245901 is the terms of A245898 with odd indices. A245888 is the number of increasing unary-binary trees whose breadth-first reading word avoids 231.

Sequence in context: A049130 A101488 A279544 * A230662 A206464 A089429

Adjacent sequences:  A245895 A245896 A245897 * A245899 A245900 A245901

KEYWORD

nonn,more

AUTHOR

Manda Riehl, Aug 05 2014

STATUS

approved

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Last modified June 25 22:15 EDT 2017. Contains 288730 sequences.