Relation reduction and relational reducibility have to do with the extent to which a given relation is determined by an indexed family or a sequence of other relations, called the relation dataset. The relation under examination is called the reductand. The relation dataset typically consists of a specified set of other relations, simpler in some measure than the reductand, called the reduction base, plus a specified operation on relations, called the reduction method or the reduction step.
A question of relation reduction or relational reducibility is sometimes posed as a question of relation reconstruction or relational reconstructibility, since a useful way of stating the question is to ask whether the reductand can be reconstructed from the relational dataset.
A relation that is not uniquely determined by a particular relation dataset is said to be irreducible in just that respect. A relation that is not uniquely determined by any relation dataset in a particular class of relation datasets is said to be irreducible in respect of that class.
The main thing that keeps the general problem of relational reducibility from being fully well-defined is that one would have to survey all of the conceivable ways of “getting new relations from old” in order to say precisely what is meant by the claim that the relation is reducible to the set of relations . This is tantamount to claiming that if one is given a set of “simpler” relations , for indices in some set , that this collection of data would somehow or other fix the original relation that one is seeking to analyze, to determine, to specify, or to synthesize.
In practice, however, apposite discussion of a particular application typically settles on either one of two different notions of reducibility as capturing the pertinent issues, namely:
2 Projective reducibility of relations
It is convenient to begin with the projective reduction of relations, partly because this type of reduction is simpler and more intuitive (in the visual sense), but also because a number of conceptual tools that are needed in any case arise quite naturally in the projective setting.
The work of intuiting how projections operate on multidimensional relations is often facilitated by keeping in mind the following sort of geometric image:
For any subset of the index set , there is the corresponding subfamily of sets, , and there is the corresponding cartesian product over this subfamily, notated and defined as .
For any point in , the projection of on the subspace is notated as , or still more simply as
More generally, for any relation , the projection of on the subspace is written as , or still more simply as
The question of projective reduction for -adic relations can be stated with moderate generality in the following way:
Given a set of -place relations in the same space and a set of projections from to the associated subspaces, do the projections afford sufficient data to tell the different relations apart?
3 Projective reducibility of triadic relations
See main entry (http://planetmath.org/TriadicRelation) on triadic relations.
By way of illustrating the different sorts of things that can occur in considering the projective reducibility of relations, it is convenient to reuse the four examples of 3-adic relations that are discussed in the main entry on that subject.
3.1 Examples of projectively irreducible relations
The 3-adic relations and are shown in the next two Tables:
0 0 0 0 1 1 1 0 1 1 1 0
0 0 1 0 1 0 1 0 0 1 1 1
Viewed in terms of operations on relational data tables, a 2-adic projection of a 3-adic relation is the 2-adic relation that results from deleting one column of the table for and then deleting all but one row of any resulting rows that happen to be identical in content. In other words, the multiplicity of any repeated row is ignored.
In the case of the above two relations, , the 2-adic projections are indexed by the columns or domains that remain, as shown in the following Tables.
0 0 0 0 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 1 1 0 1 0
0 0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 0 0 1 1 1 1 1 1
It is clear on inspection that the following three equations hold:
These equations say that and cannot be distinguished from each other solely on the basis of their 2-adic projection data. In such a case, either relation is said to be irreducible with respect to 2-adic projections. Since reducibility with respect to 2-adic projections is the only interesting case where it concerns the reduction of 3-adic relations, it is customary to say more simply of such a relation that it is projectively irreducible, the 2-adic basis being understood. It is immediate from the definition that projectively irreducible relations always arise in non-trivial multiplets of mutually indiscernible relations.
3.2 Examples of projectively reducible relations
The 3-adic sign relations and are shown in the next two Tables:
= Sign Relation of Interpreter A Object Sign Interpretant
= Sign Relation of Interpreter B Object Sign Interpretant
In the case of the two sign relations, , the 2-adic projections are indexed by the columns or domains that remain, as shown in the following Tables.
Object Sign Object Interpretant Sign Interpretant
Object Sign Object Interpretant Sign Interpretant
It is clear on inspection that the following three inequations hold:
These inequations say that and can be distinguished from each other solely on the basis of their 2-adic projection data. But this is not enough to say that either one of them is projectively reducible to their 2-adic projection data. To say that a 3-adic relation is projectively reducible in that respect, one has to show that it can be distinguished from every other 3-adic relation on the basis of the 2-adic projection data alone.
In other words, to show that a 3-adic relation on is reducible or reconstructible in the 2-adic projective sense, it is necessary to show that no distinct on exists such that and have the same set of projections. Proving this takes a much more comprehensive or exhaustive investigation of the space of possible relations on than looking merely at one or two relations at a time.
Fact. As it happens, each of the relations and is uniquely determined by its 2-adic projections. This can be seen by following the proof that is given below.
Before tackling the proof, however, it will speed things along to recall a few ideas and notations from other articles.
If is a relation over a set of domains that includes the domains and , then the abbreviated notation can be used for the projection .
Proof. Let be either one of the relations or . Consider any coordinate position in the -plane . If is not in then there can be no element in , therefore we may restrict our attention to positions in , knowing that there exist at least elements in , and seeking only to determine what objects exist such that is an element in the fiber of . In other words, for what in is in the fiber ? Now, the circumstance that has exactly one element for each coordinate in and that has exactly one element for each coordinate in , plus the “coincidence” of it being the same at any one choice for , tells us that has just the one element over each point of . All together, this proves that both and are reducible in an informative sense to 3-tuples of 2-adic relations, that is, they are projectively 2-adically reducible.
The projective analysis of 3-adic relations, illustrated by means of concrete examples, has been pursued just far enough at this point to state this clearly demonstrated result:
Some 3-adic relations are, and other 3-adic relations are not, reducible to, or reconstructible from, their 2-adic projection data. In short, some 3-adic relations are projectively reducible and some 3-adic relations are projectively irreducible.
|Date of creation||2013-10-24 16:56:16|
|Last modified on||2013-10-24 16:56:16|
|Owner||Jon Awbrey (15246)|
|Last modified by||Jon Awbrey (15246)|
|Author||Jon Awbrey (15246)|