# If $\mathbb{R}^n=\operatorname{span}\{X_1,X_2,…,X_k\}$ and $A$ is an $m\times n$ matrix, does $AX_i$ not equal $0$ for some $i$?

If $\mathbb{R}^n=\operatorname{span}\{X_1,X_2,...,X_k\}$ and $A$ is an $m\times n$ matrix, does $AX_i$ not equal $0$ for some $i$?

I think the answer might be no. But when I tried to prove it, I notice the way I did is not so persuasive. Can anybody help?

-
Are there any conditions on $A$? – Dylan Moreland Jan 27 '12 at 3:47
NO, just an m*n matrix. – Shannon Jan 27 '12 at 3:51
Do you know of any $m \times n$ matrices that send everything to zero? – Dylan Moreland Jan 27 '12 at 3:53
Do you mean 0 matrix? This question would be meaningless if A is 0 matrix. – Shannon Jan 27 '12 at 4:43

## 1 Answer

Note that if $AX_i=0$ for all $i$ then $A(a_1X_1+...+a_kX_k)=0$. Thus, as $span\{X_1,...,X_k\}=\mathbb R^n$, it follows that $AX=0$ for all $X\in \mathbb R^n$. Therefore $AX_i=0$ for all $i$ if and only if $A$ is the zero-matrix.

-
The question is that is it possible for SOME i AX_i could not equal 0? Assume we can pick arbitrary nonzero X_i from R^n,but What I am concerned is that the product of two matrices also could be zero. – Shannon Jan 27 '12 at 4:49
If $A$ is the zero-matrix then clearly all $AX_i$ are zero but for any other matrix it is true. – azarel Jan 27 '12 at 4:53
Talked to the prof today that he made some revision and stated A should be nonzero matrix. Thank you all. – Shannon Jan 27 '12 at 22:14