1. ## Basis and Dim

Let $\mathbb{F}$ be any field. Let $K_n$ be the set of all $n \times n$ matrices $A$ with entries from $\mathbb{F}$ satisfying $\sum_{i=1}^n a_{ij} = 0$ for each $j$. How do I find a basis for $K_n$ and how do I find dim $(K_n)$ ?

I really need help understanding how to find a basis and dim for any question like this.

2. Originally Posted by funnyinga
Let $\mathbb{F}$ be any field. Let $K_n$ be the set of all $n \times n$ matrices $A$ with entries from $\mathbb{F}$ satisfying $\sum_{i=1}^n a_{ij} = 0$ for each $j$. How do I find a basis for $K_n$ and how do I find dim $(K_n)$ ?

I really need help understanding how to find a basis and dim for any question like this.
for any $1 \leq i,j \leq n$ define $e_{ij}$ to be the $n \times n$ matrix with (i,j)-entry 1 and everywhere else 0. now put $a_{nj}=-\sum_{i=1}^{n-1}a_{ij}$ in $A$ to see that $A=\sum_{j=1}^n \sum_{i=1}^{n-1} a_{ij}(e_{ij}-e_{nj}).$

hence $\mathcal{B}=\{e_{ij}-e_{nj}: \ \ 1 \leq i \leq n-1, \ 1 \leq j \leq n \}$ is a basis for $K_n$ and therefore $\dim K_n=n(n-1).$