OFFSET
0,3
COMMENTS
This automorphism either swaps (if A057515(n) > 1) the first two toplevel elements (of a general plane tree, like *A072796 does) and otherwise (if n > 1, A057515(n)=1) swaps the sides of the left hand side subtree of the S-expression (when viewed as a binary tree, like *A089854 does). This is illustrated below, where letters A, B and C refer to arbitrary subtrees located on those nodes and () stands for an implied terminal node.
...B...C.............A...C............A...B...........B...A
....\./...............\./..............\./.............\./
.A...x.....-->.....B...x................x..()....-->....x..()
..\./...............\./..................\./.............\./
(a . (b . c)) --> (b . (a . c)) / ((a . b) . ()) --> ((b . a) . ())
LINKS
PROG
CROSSREFS
KEYWORD
nonn
AUTHOR
Antti Karttunen, Oct 11 2006
STATUS
approved