OFFSET
0,3
COMMENTS
.A...B...........C...A
..\./.............\./
...x...C....-->....x...B...............()..A.........()..A..
....\./.............\./.................\./....-->....\./...
.....x...............x...................x.............x....
((a . b) . c) -> ((c . a) . b) _____ (() . a) ---> (() . a)
In terms of S-expressions, this automorphism rotates caar, cdar and cdr of an S-exp, i.e., if car-side is not ().
See the Karttunen OEIS-Wiki link for a detailed explanation how to obtain a given integer sequence from this definition.
LINKS
PROG
(Scheme functions implementing this automorphism on list-structures/S-expressions, both constructive (*A089857) and destructive (*A089857!) versions:)
(define (*A089857 s) (if (and (pair? s) (pair? (car s))) (cons (cons (cdr s) (caar s)) (cdar s)) s))
(define (*A089857! s) (cond ((not (pair? s)) s) ((not (pair? (car s))) s) (else (swap! s) (robl! s) s)))
(define (swap! s) (let ((ex-car (car s))) (set-car! s (cdr s)) (set-cdr! s ex-car) 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))
CROSSREFS
KEYWORD
nonn
AUTHOR
Antti Karttunen, Nov 29 2003
EXTENSIONS
Further comments and constructive implementation of Scheme-function (*A089857) added by Antti Karttunen, Jun 04 2011
STATUS
approved