Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

One way to generate a metric for a set $S$ (a distance function between elements $a,b$ of the set $S$) would be by associating it with a vector space $V$ (the vectors that connect the elements $a,b$) and using the norm (length function) of the vectors as the distance metric for $S$.

What would be the proper term for the entity that establishes the association between the set and the vector space, i.e. for a function $f: (a,b) \Rightarrow V$?

I guess I am looking for the appropriate synonym for a term like "vectorizer" or "vector space associator".

Example: given a set $S$ of strings over some alphabet, one can define a vector space $T$ of string transforms whose elements are mappings $f: (S) \Rightarrow S$. Given a norm (e.g. number of edit operations inside the transform) on $T$ via $n: (T) \Rightarrow R$ we can then induce a metric on $S$. But to do that, we first need a mapping $f: (S, S) \Rightarrow T$ to get from the set $S$ to the vectors $T$. What is the proper term for such a mapping?

share|improve this question
1  
What is the vector space you associate to the set $S$? A natural candidate would be the Hilbert space of all square-summable sequences over the set $S$. Then to $a,b\in S$ you associate the vector that is the difference between the basis elements corresponding to $a$ and $b$. But this is by no means the only choice. You will have to clarify your question. –  Stefan Geschke Nov 26 '11 at 7:12
    
@StefanGeschke Hm, any suitable vector space, I guess, depending on the set S. Two relevant examples would be Euclidean vectors and transform functions f: a => b. –  Frank Abagnale Nov 26 '11 at 7:15
1  
If you say euclidean vectors, do you consider $S$ to be a subset of some $\mathbb R^n$? I have no idea what you mean by the transform functions. –  Stefan Geschke Nov 26 '11 at 7:17
    
@StefanGeschke Sorry. On Euclidean vectors: yes. On transform functions: I'm thinking about computations such as string transforms, f : (String,String) => StringTransform, with some associated norm n : (StringTransform) => R (don't know how to do the fancy real-numbers R). –  Frank Abagnale Nov 26 '11 at 7:32
    
Can you give an example of a set of strings and a vector space of transformations of them? I don't think I understand the question. (For instance, what would it mean to multiply a string transformation by negative pi?) –  Daniel McLaury Nov 26 '11 at 8:58

1 Answer 1

"Vector space" is a term denoting a certain kind of mathematical structure: A set $S$ provided with an addition and a multiplication by "scalars" such that certain rules hold. The elements of such a space, whatever they are (numbers, strings, functions, etc.) are called vectors. Vectors have nothing to do with arrows per se – it is just a historical coincidence that the first vectors encountered by mathematicians were the forces arrows drawn by physicists.

In your case it seems that in the first place you have arrows representing a map $f: S\to S$. You can use these arrows to create a directed graph $\Gamma$ with vertex set $S$ and the given arrows as edges. When it comes to defining a metric on $S$ one should forget about the direction of these edges to make the distance function symmetric. On the other hand you may assign positive weights to the edges accounting for some (computation) cost.

The graph $\Gamma$ is connected if for any two $x$, $y\in S$ there is an edge-path connecting $x$ and $y$ in the obvious way. Any path connecting $x$ and $y$ has a total cost, and a natural distance function in your setting would be $d(x,y):=$ minimal cost to get from $x$ to $y$.

share|improve this answer
    
Right, thank you. This gives me a richer view on the topic, which is much appreciated (I'm not a mathematician). Unfortunately, the original question is still open: given a set S, is there a proper term for the process of (or the mapping resulting from) defining a space of arrows for it so that I can use the length of the arrows as a norm on S? Not that my life depends on it :-), but I'm writing some code where this is a recurring task (given a set, find a space of transforms to get a norm) and I'm trying to map what I'm doing to existing concepts and terms where possible. –  Frank Abagnale Nov 29 '11 at 9:40
    
Frank, are you talking about something like "edit distance"? –  Daniel McLaury Nov 30 '11 at 0:48
    
Yes and No :-) The edit distance would be the norm for the edits (= transforms/vectors/arrows) and the metric for the edited strings. I want to know: is there a term for getting from the set of Strings to the space of Edits? There are two different spaces/sets here, and one is related to the other, and this is a generic concept (generating a space of transforms for a given set). So I was guessing there must be a term like "vectorizing" a set or whatever. Apparently, there is not, at least not a common one... –  Frank Abagnale Dec 1 '11 at 4:25

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.