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A089856
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Involution of natural numbers induced by Catalan Automorphism *A089856 acting on the binary trees/parenthesizations encoded by A014486/A063171.
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12
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0, 1, 2, 3, 4, 5, 8, 7, 6, 9, 10, 11, 12, 13, 21, 22, 20, 17, 18, 19, 16, 14, 15, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 58, 59, 62, 63, 64, 57, 61, 54, 45, 46, 55, 48, 49, 50, 56, 60, 53, 44, 47, 51, 42, 37, 38, 52, 43, 39, 40, 41, 65, 66, 67, 68, 69, 70, 71, 72
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OFFSET
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0,3
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COMMENTS
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This automorphism 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...........C...B
..\./.............\./
...x...C....-->....x...A...............()..A.........()..A..
....\./.............\./.................\./....-->....\./...
.....x...............x...................x.............x....
((a . b) . c) -> ((c . b) . a) _____ (() . a) ---> (() . a)
In terms of S-expressions, this automorphism swaps caar and cdr of an S-exp if possible, i.e., if car-side is not ().
See the Karttunen OEIS-Wiki link for a detailed explanation of how to obtain a given integer sequence from this definition.
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LINKS
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PROG
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(Scheme function implementing this automorphism on list-structures/S-expressions, both constructive (*A089856) and destructive (*A089856!) versions:)
(define (*A089856 s) (if (and (pair? s) (pair? (car s))) (cons (cons (cdr s) (cdar s)) (caar s)) s))
(define (*A089856! s) (cond ((not (pair? s)) s) ((not (pair? (car s))) s) (else (let ((org_caar (caar s))) (set-car! (car s) (cdr s)) (set-cdr! s org_caar) s))))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Further comments and constructive implementation of Scheme-function (*A089856) added by Antti Karttunen, Jun 04 2011
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STATUS
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approved
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