# Finding basis of spaces and dimension

## Homework Statement

Find a basis for each of the spaces and determine its dimension:
The space of all matrices A=[a b, c d] (2x2 matrix) in R^(2x2) such that a+d=0

## The Attempt at a Solution

So I jumped at this question without knowing too much about spaces and dimensions, but:
I think a possible combination of basis can be: [1 0, 0 0]. [0 1, 0 0]. [0 0, 1 0] (not sure if [0 0, 0 -1] would be considered since d would be negative in this case) Also from that I assume the dimension is 3?
Truthfully even if I got it right, I do not even know what happened. Can someone conceptually tell me what I am doing exactly?

lanedance
Homework Helper
you need to come up with a basis, that has elements, such that any 2x2 matrix with a=d can be written as a linear sum of the basis elements.

clearly the basis elements will need to satisfy being a 2x2 matrix with d=a, you first element does not

lanedance
Homework Helper
note this is entirely equivalent to considering 4 component vectors in R^4, with x_1 = x_4

often the vector form is easier to conceptualise

Last edited:
you need to come up with a basis, that has elements, such that any 2x2 matrix with a=d can be written as a linear sum of the basis elements.

clearly the basis elements will need to satisfy being a 2x2 matrix with d=a, you first element does not

Can you explain to me how d=a, since a+d=0?
Also since I am actually having a hard time learning this on my own, can you tell me what you mean by "elements" and "linear sum"?

lanedance
Homework Helper
Can you explain to me how d=a, since a+d=0?
Also since I am actually having a hard time learning this on my own, can you tell me what you mean by "elements" and "linear sum"?
good pickup, should be a = -d

if you're not familair with those terms, you may need to do a bit of reading.. though i have been a little loose with terminiology

in post #4 i actually meant component and have changed accordingly

a basis, is a set of vectors that spans a vector space

an element is a member of a set, for example a vector in the basis set

a linear combination (or sum) is a vector addition with scalar multiplication
eg. if u,v are vectors, and a,b are scalars, then
w = au + bv is a linear combination

a set, S, of vectors spans a space if any vector in the space can be written as a linear combination of vectors in S

HallsofIvy
$$\begin{bmatrix}a & b \\ c & d\end{bmatrix}= \begin{bmatrix}a & b \\ c & -a\end{bmatrix}$$
$$= \begin{bmatrix}a & 0 \\ 0 & -a\end{bmatrix}+ \begin{bmatrix}0 & b \\ 0 & 0\end{bmatrix}+ \begin{bmatrix}0 & 0 \\ c & 0\end{bmatrix}$$