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A072797
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Self-inverse permutation of natural numbers induced by a Catalan bijection acting on binary trees as encoded by A014486. See comments and examples for details.
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33
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0, 1, 2, 3, 4, 5, 7, 6, 8, 9, 10, 11, 12, 13, 17, 18, 16, 14, 15, 20, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 45, 46, 48, 49, 50, 44, 47, 42, 37, 38, 43, 39, 40, 41, 54, 55, 53, 51, 52, 57, 56, 58, 59, 61, 60, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71
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OFFSET
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0,3
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COMMENTS
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This bijection effects the following transformation on the unlabeled rooted plane binary trees (letters A, B, C refer to arbitrary subtrees located on those nodes and () stands for an implied terminal node).
A B A C
\ / \ /
x C --> x B () A () A
\ / \ / \ / --> \ /
x x x x
((a . b) . c) --> ((a . c) . b) (() . a) ---> (() . a)
See the example for an explanation of how to obtain a given integer sequence from this definition.
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LINKS
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FORMULA
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EXAMPLE
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To obtain the signature permutation, we apply these transformations to the binary trees as encoded and ordered by A014486 and for each n, a(n) will be the position of the tree to which the n-th tree is transformed to, as follows:
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one tree of one internal
empty tree (non-leaf) node
x \/
n= 0 1
a(n)= 0 1 (both are always fixed)
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the next 7 trees, with 2-3 internal nodes, in range [A014137(1), A014137(2+1)-1] = [2,8] are:
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\/ \/ \/ \/
\/ \/ \/ \/ \/ \/ \/ \/
\/ \/ \/ \/ \_/ \/ \/
n= 2 3 4 5 6 7 8
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and the new shapes after swapping the two subtrees in positions marked "B" and "C" in the diagram given in the comments are:
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\/ \/ \/ \/
\/ \/ \/ \/ \/ \/ \/ \/
\/ \/ \/ \/ \/ \_/ \/
a(n)= 2 3 4 5 7 6 8
thus we obtain the first nine terms of this sequence: 0, 1, 2, 3, 4, 5, 7, 6, 8.
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PROG
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(Scheme function implementing this automorphism on list-structures/S-expressions, both constructive (*A072797) and destructive (*A072797!) version:)
(define (*A072797 s) (if (and (pair? s) (pair? (car s))) (cons (cons (caar s) (cdr s)) (cdar s)) s))
(define (*A072797! s) (cond ((not (pair? s)) s) ((not (pair? (car s))) s) (else (swap! s) (robl! s) (swap! (car s)) s)))
(define (robl! s) (let ((ex-car (car s))) (set-car! s (cddr s)) (set-cdr! (cdr s) ex-car) (swap! (cdr s)) (swap! s) s))
(define (swap! s) (let ((ex-car (car s))) (set-car! s (cdr s)) (set-cdr! s ex-car) s))
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CROSSREFS
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Counts for the fixed points and for the number of distinct cycles (in each subrange limited by A014137) are given by A073190 and A073191.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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