# Linear Algebra - Vector Subspaces

1. Feb 21, 2008

### steelphantom

1. The problem statement, all variables and given/known data
Give an example of a nonempty subset U of R2 such that U is closed under scalar multiplication, but U is not a subspace of R2.

2. Relevant equations

3. The attempt at a solution
I think I have it, but I just want to make sure it's right:

Let U = {(x, x + 2)} | x is in R}. Let u be in U. Then au = a(x, x + 2) = (ax, a(x + 2)), which is still in U (i.e. still on the same line). But this is not a subspace of R2 because 0 is not in this subset.

Thanks!

2. Feb 21, 2008

### NateTG

You get $<0,0>$ if you multiply by $a=0$.
(Hint: What are the properties of subspaces?)

3. Feb 21, 2008

### steelphantom

1) Additive Identity: 0 is in U.

2) Closed Under Addition: u, v in U implies u + v is in U.

3) Closed Under Scalar Multiplication: a is in R (or C) and u is in U implies au is in U.

So I need to come up with a subset that fails for one (or both) of the first two properties, but not for property 3.

How about checking for property 2? If u = (x, x + 2) and v = (y, y + 2), where u, v are in U, then u + v = (x + y, (x + y) + 4), which is not in U, so U is not a subspace of R2.

4. Feb 21, 2008

### NateTG

...better, but:
$$2<3,(3+2)>=<6,10> \notin U$$
so you don't have closure under scalar multiplication.

5. Feb 21, 2008

### steelphantom

Wow, I guess I don't. I really don't know where to go from here. Any hints on a a type of subset that would work in this situation?

6. Feb 21, 2008

### steelphantom

I was searching the forums and found pretty much the exact same question. Here's the thread if anyone is interested: https://www.physicsforums.com/showthread.php?t=41884"

A set that works is the following: U = {(x, y) | xy = 0}. This set is closed under scalar multiplication, but not vector addition.

Last edited by a moderator: Apr 23, 2017
7. Jan 31, 2010

### studguy

You are absolutely wrong.U={(x,x+2)| x is in R} is not closure under multiplication.
Ex:put x=0,a=2 (0,2) is in U but (0,4) is not in U.

8. Jan 31, 2010

### HallsofIvy

Staff Emeritus
Yes. he was told that almost two years ago!