# Empty Set and Vector Space

1. Aug 3, 2005

### HungryChemist

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?

2. Aug 3, 2005

### James R

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

3. Aug 3, 2005

### 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?

4. Aug 3, 2005

### symplectic_manifold

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)

5. Aug 3, 2005

### Galileo

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

6. Aug 3, 2005

### matt grime

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.

7. Aug 3, 2005

### symplectic_manifold

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?

8. Aug 3, 2005

### matt grime

of course it does.

9. Aug 4, 2005

### HungryChemist

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

10. Aug 4, 2005

### 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 V but there are no such element if the set S is empty. No?

Last edited: Aug 4, 2005
11. Aug 4, 2005

### matt grime

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

12. Aug 4, 2005

### 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!

13. Aug 5, 2005

### HungryChemist

shoudn't it be $\emptyset=\{x\in{M}|x\neq{y}\}$?

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

14. Aug 5, 2005

### 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.

15. Aug 5, 2005

### hypermorphism

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).

16. Aug 6, 2005

### HungryChemist

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

17. Aug 11, 2005

### 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.

18. Aug 11, 2005

### HallsofIvy

Staff Emeritus
In what sense does the span of the empty set equal the set containing the 0 vector?

19. Aug 11, 2005

### Hurkyl

Staff Emeritus
"The span of a set of vectors is the smallest subspace containing those vectors."

:tongue2:

Also, the zero vector is the linear combination of no vectors. (i.e. the "empty sum")

20. Aug 11, 2005

### mathwonk

the summation symbol is a function from indexed sets of vectors to the vector space, which is additive on disjoint decompositions of the index set. hence it must send the empty index set (which is an indexed set, since the empty set is a set) to zero.

hence the empty linear combination must equal zero. Many many proofs in linear algebra are lacunary without this remark. I just taught it last semester and found out how hard it is to ignore this case without making mistakes, (which however no one noticed except me). Indeed even the book I used made this mistake - although the authors tried hard to give correct statements without pointing out this subtle fact, they did not quite succeed. :rofl:

Last edited: Aug 11, 2005