gatomorphism f embeds into gatomorphism g in scale n:mif there is an injection:
e: Catn (parenthesizations/trees/etc. of size n) --> Catm (parenthesizations/trees/etc. of size m)that for all the elements a of the former set, and for all integer values of the superscript r in Z (which stands for the repeated applications of f or g (if the exponent is positive) or their inverse (if the exponent is negative)) it holds that
(gr (e a)) = (e (fr a))
gatomorphism g is Lukasiewicz-word permutingif the Lukasiewicz-word of (g s) is always a permutation of the Lukasiewicz-word of s for all trees/parenthesizations s of any size.
This implies that the subset of the planar binary trees (of the general trees) is closed under the action of g, and thus its restriction to that subset induces another gatomorphism f. Rephrasing this in above terms, we can say that gatomorphism f embeds into gatomorphism g in scale n:2n.
gatomorphism g is self-embeddableif g embeds into itself in scale n:m, where n ranges through all values in N, and m is a function of n, with m > n.
Two special cases of self-embeddability deserve special attention:
Definition. We say that
gatomorphism g is "horizontally telescoping"if for all the parenthesizations s it holds that
(g (cons '( ) s)) = (cons '( ) (g s))where the injection e required by our general definition of embeddability is now defined as
(define (e s) (cons '( ) s))which maps the parenthesizations/trees of size n to the parenthesizations/trees of size n+1 by simply concatenating empty parentheses to the front of the corresponding parenthesization, or in terms of binary trees, by grafting an old tree of size n as a right-hand-side subtree of a new tree of size n+1 whose left-hand-side subtree is just a single edge \.
Remark.
This is equivalent to saying that each sub-permutation of the length A000108(n) induced by g's action on
the standard sequence of lexicographically ordered parenthesizations
A014486 (A063171)
starts with the same cycle-structure as the previous sub-permutation.
In this case we can form yet another permutation of the natural numbers
by conceptually taking the "infiniteth" of such sub-permutations
and by "normalizing" it to begin from 0 or 1.
For example, Meeussen's breadth-first <-> depth-first conversion for binary trees
A057117
satisfies the above condition, and by thus "contracting the telescope" we get another
permutation of natural numbers,
A038776.
Definition. We say that
gatomorphism g is "vertically telescoping"if for all parenthesizations s it holds that
(g (cons s '( ))) is equal to (cons (g s) '( ))where the injection e is now defined as
(define (e s) (cons s '( )))which maps the parenthesizations/trees of size n to the parenthesizations/trees of size n+1 by surrounding the corresponding parenthesization with one extra parentheses ( and ), or in terms of binary trees, by grafting an old tree of size n as a left-hand-side subtree of a new tree of size n+1 whose right-hand-side subtree is just a single edge /.
Remark. For example, it is easy to see that this holds for the gatomorphism A057164 (Deep Reverse), and similarly for A057511/A057512 (Deep Rotates), and as A069787 = A057163 o A057164 o A057163, we can contract A069787 (a horizontally telescoping gatomorphism) to get A072799.
Remark. The allusions "horizontal" and "vertical" should be obvious if one thinks in the terms of Dyck paths (Catalan Mountain Ranges), and also the plane general trees in the latter case.