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 HungryChemist Aug3-05 02:11 AM

Empty Set and Vector Space

it reads, The empty set( a vector space with no elements) is denoted as & (This symbol doesn't matter for the sake of argument, I don't know how to write the Zero with a line in the middle). I can understand what they mean by empty set. It must be somewhat like this; Set V = { }. But can such set with no elements has all the quality of being a vector space? If so, how can one show it does indeed meets all the ten axioms of vector space?

 James R Aug3-05 02:37 AM

It seems to me that a vector space ought to contain vectors, so the empty set probably doesn't qualify.

 matt grime Aug3-05 03:42 AM

The thing with maths is that the empty set can be a FOO since there are no elements of the empty set for whcih the definitions of FOO are false. BUt, that said, I don't think this case is one of them, there is for instance no additive identity. Which text is this?

 symplectic_manifold Aug3-05 03:57 AM

Hm
Since the empty set is a subset of every set, I think one can still say it's a vector space.
The statement $$x\in\emptyset$$ is always wrong, so you can derive any statment you wish, in this case the axioms for vector spaces, which will apply to the empty vector space. (In fact, everything can follow from a wrong statement)

 Galileo Aug3-05 04:06 AM

Vector spaces need a zero vector (an additive identity) just like groups need an identity element. So empty sets cannot be vector spaces.

 matt grime Aug3-05 04:18 AM

Of course, almost every time I define these things for students I declare the underlying set to be not empty and that removes any doubt.

 symplectic_manifold Aug3-05 04:37 AM

OK, I looked it up.
The definition presupposes that a subspace of a vector space be non-empty. But does a vector space deserve to be called a set then?

 matt grime Aug3-05 06:17 AM

of course it does.

 HungryChemist Aug4-05 01:49 AM

Quote:
 Quote by matt grime The thing with maths is that the empty set can be a FOO since there are no elements of the empty set for whcih the definitions of FOO are false. BUt, that said, I don't think this case is one of them, there is for instance no additive identity. Which text is this?

This is yet published text which my professor uses for my classes. This class is Mathematical Methods/Numerical Analysis.

 HungryChemist Aug4-05 01:53 AM

Quote:
 Quote by symplectic_manifold Hm Since the empty set is a subset of every set, I think one can still say it's a vector space.
Empty set is a subset of every set can't be true. If a set S is a subset of a set V then there should be at least one element of set S that also belongs to a set V but there are no such element if the set S is empty. No?

 matt grime Aug4-05 02:10 AM

I'm afraid the empty set is a subset of any set, at least in any (model of a) set theory worth its salt.

 symplectic_manifold Aug4-05 02:32 AM

Quote:
 Quote by HungryChemist Empty set is a subset of every set can't be true. If a set S is a subset of a set V then there should be at least one element of set S that also belongs to a set S but there are no such element if the set S is empty. No?
Well, this is it.
The empty set is a subset of every set exactly because of the fact, that one doesn't need to verify, that every element of the empty set also belongs to a non-empty set.
If we have a property which no elements of a non-empty set have, we obtain the empty subset of this non-empty set:
$\emptyset=\{x\in{M}|x\neq{x}\}$

...but as I eventually made clear for myself, it has nothing to do with a vector space...nothing can be defined on an empty set...from nothing comes nothing! o:)

 HungryChemist Aug5-05 02:20 AM

Quote:
 Quote by symplectic_manifold Well, this is it. The empty set is a subset of every set exactly because of the fact, that one doesn't need to verify, that every element of the empty set also belongs to a non-empty set. If we have a property which no elements of a non-empty set have, we obtain the empty subset of this non-empty set: $\emptyset=\{x\in{M}|x\neq{x}\}$ ...but as I eventually made clear for myself, it has nothing to do with a vector space...nothing can be defined on an empty set...from nothing comes nothing! o:)
shoudn't it be $\emptyset=\{x\in{M}|x\neq{y}\}$?

x not equal x sounds very wrong.....

 Galileo Aug5-05 02:54 AM

No, it's correct. $x \not= x$ is always false: there is no x which satisfies that nonequality, therefore the given set is empty. You didn't even specify what y is, so your expression has no meaning.

To relieve any doubt, use the following definition of subset:

If A and B are sets, then A is called a subset of B if:
$$x \in A \Rightarrow x \in B$$

the notation is $A \subset B$.

Do you see now why the empty set is a subset of every set.

 hypermorphism Aug5-05 11:47 AM

Also note that this property conveniently gives us the property that the intersection of two sets which have no elements in common is still a set, the empty set (since it is a subset of both sets).

 HungryChemist Aug6-05 01:26 AM

Quote:
 Quote by Galileo No, it's correct. $x \not= x$ is always false: there is no x which satisfies that nonequality, therefore the given set is empty. You didn't even specify what y is, so your expression has no meaning. To relieve any doubt, use the following definition of subset: If A and B are sets, then A is called a subset of B if: $$x \in A \Rightarrow x \in B$$ the notation is $A \subset B$. Do you see now why the empty set is a subset of every set.

Yes! Thank you very much. I am always amazed!

 PBRMEASAP Aug11-05 05:12 AM

Quote:
 Quote by HungryChemist While reading text, I had a question which I can not resolve by myself. Please Help me! it reads, The empty set( a vector space with no elements) is denoted as & (This symbol doesn't matter for the sake of argument, I don't know how to write the Zero with a line in the middle). I can understand what they mean by empty set. It must be somewhat like this; Set V = { }. But can such set with no elements has all the quality of being a vector space? If so, how can one show it does indeed meets all the ten axioms of vector space?
Yeah, as others pointed out, the empty set can't be a vector space because it has no zero vector. However, the empty set does span the vector space consisting of the zero vector, according to the definition of span: The span of a set of vectors is the smallest subspace containing those vectors.

 HallsofIvy Aug11-05 07:10 AM

Quote:
 Quote by PBRMEASAP Yeah, as others pointed out, the empty set can't be a vector space because it has no zero vector. However, the empty set does span the vector space consisting of the zero vector, according to the definition of span: The span of a set of vectors is the smallest subspace containing those vectors.
In what sense does the span of the empty set equal the set containing the 0 vector?

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