# Subspace for R^3

• Apr 21st 2008, 06:58 PM
pakman
Subspace for R^3
{(x1, x2, x3)^T | x3 = x1 or x3 = x2}

I know it is not a subspace of R^3, however I'm not exactly sure why. I'm thinking it is because of the OR in the statement? In an earlier problem I had to prove that the same set and statement x1 = x2 = x3 is a subspace, which held true. But this OR is throwing me off. Any ideas?
• Apr 21st 2008, 07:13 PM
TheEmptySet
Quote:

Originally Posted by pakman
{(x1, x2, x3)^T | x3 = x1 or x3 = x2}

I know it is not a subspace of R^3, however I'm not exactly sure why. I'm thinking it is because of the OR in the statement? In an earlier problem I had to prove that the same set and statement x1 = x2 = x3 is a subspace, which held true. But this OR is throwing me off. Any ideas?

This is not a subspace it is not closed

Example

consider $v_1=(1,2,1) \mbox{ and } v_2=(-1,-2,-2)$

both $v_1,v_2$ fit the above criteria but

$v_1+v_2=(1,2,1)+(-1,-2,-2)=(0,0,-1)$

This is not in the "subspace" because $x_3=-1$

but both $x_1 \mbox{ and } x_2=0$

(Rock)