Göbel = Matula Numbering. (Neé: Riffs & Rotes & A061396)
Jon Awbrey
jawbrey at oakland.edu
Tue Sep 10 15:16:23 CEST 2002
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some incidental remarks on riffs & rotes,
from the standpoint of ordered domains.
suppose that we don't know anything at all about the number theory connection,
but are simply given a species of trees defined by the following recursion:
| @ is a rote.
|
| if f is a finite function from rotes to rotes,
| say, f : index_j ~> exponent_j for j = 1 to k,
| where the "indices" and "exponents" are rotes,
| then the following tree is a rote:
|
| i_1 e_1 i_k e_k
| o----o o----o
| \ ... /
| \ | /
| \ | /
| \|/
| @
|
| where the rotes for the "indices" (source elements) i_j
| and the "exponents" (target elemnts) e_j are attached
| at the places indicated.
so the question is, what is a natural way of partial ordering this species?
then, can we specify enough information to make it a total linear ordering?
jon awbrey
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AK = Antti Karttunen
JA = Jon Awbrey
JA: | F. Göbel,
| "On a 1-1-Correspondence between Rooted Trees and Natural Numbers",
| 'Journal of Combinatorial Theory', B29 (1980), pages 141-143.
JA: This correspondence is associated in one way or another
with the following sequences in the EIS online database:
A005517, A005518, A007097, A057452.
JA: In 1980, we find the above paper published,
giving a correspondence between rooted trees
and natural numbers that is dual, in a sense,
to the one that I gave between natural numbers
and planted plane trees. In summary, we have:
1. Recur on Index : N <-> Rooted Trees
2. Recur on Power : N <- Planted Plane Trees
3. Recur on Both : N -> Riffs <-> Rotes
JA: As given, the 2nd mapping is many-to-one and onto
from planted plane trees to natural numbers, but
there is a way to make the 2nd mapping injective,
at least, I have an old note that claims this,
but I will need to read it over again first,
as I'm not as sure as I used to be about it.
JA: I also notice that Göbel's paper is
given as "Received June 26, 1978",
so I begin to suspect that there
must have been something in the
air (or the water) that year.
JA: Here is a cartoon synopsis of how Göbel's numbering goes:
¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤
|
| 1 blank blank
| 2 p_1 ()
| 3 p_(p_1) (())
| 4 p_1^2 ()()
| 5 p_(p_(p_1)) ((()))
| 6 p_1 p_(p_1) ()(())
| 7 p_(p_1^2) (()())
| 8 p_1^3 ()()()
| 9 p_(p_1)^2 (())(())
| 10 p_1 p_(p_(p_1)) ()((()))
|
| ...
|
¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤
|
| o
| |
| o o o o o
| | | | \ /
| o o o o o o o o o o o
| | | \ / | \ / | \|/
| @ @ @ @ @ @ @ @
|
| 1 2 3 4 5 6 7 8 ...
|
¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤~~~~~~~~~¤
AK: Something in the air ten years earlier ?
| D.W. Matula, Washington University, St. Louis, Missouri.
| A Natural Rooted Tree Enumeration by Prime Factorization.
| SIAM Rev. 10 (1968), p. 273.
|
| A one-to-one correspondence between the space of rooted
| trees and the positive integers is given by the following
| recursive procedure. Let the positive integer n correspond
| to the root node of a tree, the prime factors of n
| (each occurring as often as its multiplicity) correspond
| to the edges incident to the root node, and for each such
| edge incident to the root which corresponds to the prime p
| let the branch of the tree incident to that edge be the
| rooted tree corresponding to the number pi(p) (the prime
| number function pi). The terminal condition of this
| recursion arises quite naturally as unity (which has no
| prime factors) is thus associated with each pendent vertex.
| On the other hand, the number associated with a given
| rooted tree may be recursively determined as follows:
| If the numbers associated with the branches of a rooted
| tree T are n1, n2, ..., nm, then the unique number
| associated with T is the product p(n1) x p(n2) x ... x p(nm),
| where p(ni) is the ni:th prime. Certain interesting
| relationships between the theories of primes and trees
| will be developed utilizing this "natural" correspondence.
AK: Compare with the beginning of Göbel's paper:
| In the sequel, we suppose that p(1) = 2, p(2), p(3), ...
| is the sequence of primes in ascending order.
| Let T be a rooted tree, r its root. The connected components
| of T - r are denoted by T1, ..., Tv, where v is the degree
| of r. The graphs Tj (j = 1,...,v) obviously are trees, which
| we transform into rooted trees by defining as the the root
| of Tj the vertex of Tj which is adjacent to r in T. We now
| assign a natural number phi(T) to the rooted tree T.
|
| Definition. If T has only one vertex, then phi(T) = 1.
| If the number of vertices of T is greater than 1, then
|
| phi(T) = product_{i=1..v} p(phi(Ti)),
|
| where T1, ..., Tv are the rooted trees defined above.
|
| ...
AK: There's at least one on-line document about Matula-numbers:
| Ivan Gutman and Yeong-Nan Yeh, Deducing properties of trees from their Matula numbers,
| Publications de L'institut Mathématique (Beograd), Vol. 53(67), pp. 17-22 (1993).
| URL: http://www.emis.de/journals/PIMB/067/n067p017.pdf
AK: It would be nice to compute the functions f1 - f10
(corresponding to the properties P1 - P10) defined in that paper.
I don't know how much you can find connections from there to the
classical elementary number theory, but as a simple example, the
degree of the rooted tree with Matula/Göbel number n is bigomega(n),
(A001222), and "the primeth recurrence" A007097 gives the trees with
just one leaf.
AK: Now, I started thinking why we must use specifically the ordinary primes.
Why not use instead the binary encoding of irreducible Z2[x] polynomials, i.e.
A014580 (2,3,7,11,13,19,25,31,...) instead of A000040 (2,3,5,7,11,13,17,19,23,...)
as the function 'p' in the above definition given by Göbel? Of course then we
should use the carryless Z2[x] multiplication instead of the ordinary one,
to acquire a one-to-one map.
AK: So, 5 = 101 in binary (corresponding to x^2 + 1 = x+1 * x+1 in Z2[X])
corresponds now to the tree:
\ /
\/
which in original Matula/Göbel scheme is associated with the number 9.
(Deducing the properties of the trees using this scheme might be
slightly easier than with the original one?)
AK: Combining the mapping N <-> rooted trees, and rooted trees <-> Z2[x] polynomials we
obtain a map between the multiplicative semigroup (or specifically: commutative monoid)
of natural numbers and the multiplicative semigroup found in the ring of Z2[x] polynomials.
Let's call it khi. Unless I'm completely zombie, it obeys
khi(1) = 1
and
khi(ab) = khi(a) X khi(b)
AK: where X means multiplication of Z2[x] polynomials,
so it is a homomorphism between these semigroups.
AK: Using the standard binary encoding of Z2[x]
polynomials we can represent it in OEIS as
yet another permutation of natural numbers.
Among other things, it maps A000040 and A014580
term-by-term to each other.
AK: (How does it map the corresponding multiplication tables
A004247 and A048720 to each other?)
AK: Then we can get even wilder, because any domain
with unique factorization can be used, so why not to
use, say Gaussian integers, or some other "double ring"?
I remember that Marc spoke some months ago about storing
these ones to OEIS with the "bits-interleaved" method.
JA: i will see if i can find the references later, but i remember
neil or maybe nordhaus telling me about some similar ideas
that a.a. mullin (?) called 'mosaics', some of which got
published in the nat.acad.sci., and i think that there
were related constructions going back to a goodstein
or goldstein in the 20's, also some connections to
mirimanoff. all very fuzzy now.
JA: my initial interest in riffs and rotes came out of a search for
"auto-documenting data structures" and "self-tagging molecules",
plus links between additive and multiplicative number theory that
i associated with the question: "how much of the ordering of the
integers is purely combinatorial", whatever that means. so, yes,
some flavor of generalized primes comes into play -- if you ask:
JA: is there a "purely combinatorical" reason that (p_2)^1 < (p_1)^2?
JA: viewed as rotes:
| o---o o---o
| | |
| o---o o---o
| | |
| @ <?< @
|
| (p_2)^1 <?< (p_1)^2
JA: fun & games ...
JA: links here to dana scott's d_infinity and similar constructions.
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