# Re: what are keys and surrogates?

Date: Sun, 13 Jan 2008 18:22:17 -0800 (PST)

Message-ID: <0409ba60-5834-425e-9f20-06830ed63d21_at_s12g2000prg.googlegroups.com>

On Jan 14, 7:25 am, mAsterdam <mAster..._at_vrijdag.org> wrote:

*> David BL schreef:
**>
**>
**>
*

> > Keith H Duggar wrote:

*> >> David BL wrote:
**> >>> Keith H Duggar wrote:
**> >>>> David BL wrote:
**> >>>>> Marshall wrote:
**> >>>>>> An interesting note, by the way:
**> >>>>>> functions are relations ...
**> >>>>> Isn't it more precise to say that the graph of a
**> >>>>> function is a relation?
**> >>>> No, it isn't.
**> >>>>http://mathworld.wolfram.com/Function.html
**> >>> From mathworld a relation
**> >>> http://mathworld.wolfram.com/Relation.html
**> >>> is defined as a subset of a cartesian product. If a
**> >>> function is a relation why do they define a graph of
**> >>> a function f as
**> >>> { (x,f(x)) | x in domain of f },
**> >>> as described in
**> >>> http://mathworld.wolfram.com/FunctionGraph.html
**> [snip]
**> > ... Wikipedia for example defines "graph of
**> > function" without any such restriction
**>
**> > http://en.wikipedia.org/wiki/Function_%28mathematics%29
**>
**> > http://en.wikipedia.org/wiki/Graph_of_a_function
**>
**> [snip]
**>
**> Quoted from there:
**> "In mathematics, the graph of a function f is the collection of
**> all ordered pairs (x,f(x)). In particular, graph means the
**> graphical representation of this collection, in the form
**> of a curve or surface, together with axes, etc."
**>
**> ISTM this is what I use the word 'plot' for.
**>
**> Also from that page (at the start):
**> "For another use of the term "graph" in mathematics,
**> see graph theory".
**>
**> In dutch 'grafiek' is a 'plot' (or 'chart'), and
**> 'graph' (another word) is a 'collection of edges and nodes'
**> - maybe it boils down to a homonym problem in english?
*

I was exposed to the formal notion of a graph of a function in university when I studied functional analysis. Check out the closed graph theorem

http://en.wikipedia.org/wiki/Closed_graph_theorem

This is expressed on Banach spaces and has little to do with any visualisation.

> > This however doesn't change the fact that most authors define a

*> > (mathematical) relation as a set of ordered tuples, which means a
**> > function is not a relation (assuming, as most do, that a function has
**> > a defined domain and codomain).
**>
**> ?
**>
**> How does having a domain and a codomain stops a function from being a
**> kind of relation ? (David Cressey asked a similar question).
*

Saying that a function is not a relation is not terribly insightful and I'm sorry I said it! It only has to do with what formalisms tend to predominate in the literature.

> > Furthermore, I was correct when I

*> > stated that a graph of a function is a relation, according to the more
**> > general definition of graph of function, as described in Wikipedia.
**>
**> Is it more general?
**>
**> Maybe so.
**> Anyway, what is wrong with using 'plot' for this, in order to
**> disambiguate - is there some meaning lost?
*

To some people there is an established convention to say "graph of function" to formally refer to the set of ordered pairs, irrespective of any visual rendition. Note that it is useful to have some terminology for this set, and it's not the first time that mathematical terms are overloaded. Received on Mon Jan 14 2008 - 03:22:17 CET