# Vector Space question

• Jun 20th 2008, 11:34 AM
Kim Nu
Vector Space question
Hi,

Could someone explain to me why the set of all elements of R^3 with the first component of 1 is not a real vector space? I'm lost on this one. Thanks,

Kim
• Jun 20th 2008, 11:43 AM
flyingsquirrel
Hi
Quote:

Originally Posted by Kim Nu
Could someone explain to me why the set of all elements of R^3 with the first component of 1 is not a real vector space? I'm lost on this one. Thanks,

It can't be a vector space because this set is not closed under addition :

$
\begin{pmatrix}
1\\
0\\
0
\end{pmatrix}
+\begin{pmatrix}
1\\
0\\
0
\end{pmatrix}=\begin{pmatrix}
2\\
0\\
0
\end{pmatrix}
$
• Jun 20th 2008, 11:43 AM
CaptainBlack
Quote:

Originally Posted by Kim Nu
Hi,

Could someone explain to me why the set of all elements of R^3 with the first component of 1 is not a real vector space? I'm lost on this one. Thanks,

Kim

To be a vector space all linear combinations of elements would have to be in the set, but as the sum of any two elements is not in the set it is not a vector space.

RonL
• Jun 20th 2008, 11:44 AM
o_O
Consider vectors: $\bold{u} = (1, u_{2}, u_{3})$ and $\bold{v} = (1, v_{2}, v_{3})$.

One of the axioms state that if u and v are vectors in your vector space, then so should u + v. However:
$\bold{u} + \bold{v} = (2, u_{2} + v_{2}, u_{3} + v_{3})$

which does not belong in your vector space.