Oracle FAQ | Your Portal to the Oracle Knowledge Grid |
Home -> Community -> Usenet -> comp.databases.theory -> Re: Testing for the equivalence relation
Dan wrote:
> "No, there are two equivalence classes. Period. Saying that there are 4
> is sloppy terminology."
>
> Equivalance classes, denoted with brackets:
> [a] = {a, b}
> [b] = {a, b}
> [c] = {c, d}
> [d] = {c, d}
>
> Distinct equivalence classes
> {a,b}
> {c,d}
I have 4 sisters. There is the daughter of my father, the daughter of my mother, the grand-daughter of my grand-mother and the grand-daughter of my grand-father. Since all these girls happen to be identical I have only one distinct sister.
Wouldn't you agree that the first sentence is a bit misleading? What I have 4 of is not sisters, but descriptions of my sister. What you have 4 of is not equivalence classes, but definitions of equivalence classes.
> Jan: "Er, actually, you cannot. The notion of 'reflexivity' is defined
> wrt. (1) a domain and (2) a binary relation over that domain. The notion is
> not really defined if you only have (2). Of course you could introduce
> a related notion that only needs (2), but that would be a different
> notion."
>
> So why wouldn't the set S suffice as being the basis of (1)?
It would.
> And yes, I am curious whether another notion applies here.
Sure. That's Dan-reflexivity and it's defined as follows. A binary relation is said to be Dan-reflexive if it is reflexive wrt. the union of its left-column and right column. So where do we go from here?
>>For me, one of the benfits of answering this question is that it goes a >>long way ascertaining a distinction between type and domain.
Very Zen.