Rank of m by n-matrix

linear algebra

Suppose that $V$ is spanned by $\def\R{\Bbb R}\{v_1, v_2, v_3\}\subset\R^m$ and $W$ is spanned by $\{w_1, w_2, w_3\}\subset\R^n$ with $m \geq 3$ and $n\geq3$. If $V$ is of dimension two, and $W$ is of dimension three, then what is the rank of the $m$ by $n$-matrix $A$ defined by $A=v_1w_1^T +v_2w_2^T +v_3w_3^T$?

I know that one of the $v$'s can be represented as linear combination of other two, and $w$'s are linearly independent, and I don't know what to do next. Can anyone help me?

Best Answer

Let $B$ be the $m\times 3$ matrix whose columns are $v_1,v_2,v_3$, and $C$ the $3\times n$ matrix whose rows are $w_1^T,w_2^T,w_3^T$; then $A=BC$. Now the dimensions of $V$ and $W$ tell you that the rank of $B$ is $2$ and the rank of $C$ is $3$. Then the linear map $\def\R{\Bbb R}\R^n\to\R^3$ given by $C$ has rank$~3$ and is therefore surjective, and it follows that the rank of the composite map $\Bbb R^n\to\R^3\to\R^m$ given by $BC=A$ equals the rank of $B$, that is $2$.

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