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I am working on problems from my textbook. However, I am lost as to how to show this.

A. Let $M_{2\times 2}$ be the vector space of all $2\times 2$ matrices. Show that the set of non-singular $2\times 2$ matrices is NOT a subspace of $M_{2\times 2}$.

B. Let $M_{2\times 2}$ be the vector space of all $2\times 2$ matrices. Show that the set of singular $2\times 2$ matrices is NOT a subspace of$M_{2\times 2}$.

C. Describe the smallest subspace of $M_{2\times 2}$ that contains matrices $$\begin{pmatrix}2 & 1 \\ 0 & 0\end{pmatrix},\ \ \begin{pmatrix}1 & 0 \\ 0 & 2\end{pmatrix}\ \ \text{and}\ \ \ \begin{pmatrix}0 & -1 \\ 0 & 0\end{pmatrix}$$

Find the dimension of the subspace.

I think I can prove that addition for A and B is not closed, thus disproving the potential for subspace. Though, I am not sure about C.

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1 Answer

For a, is the zero matrix in the set?

For b, show that addition is not closed (can you think of two matrices which are non-invertible but add to the identity?)

For c, notice that any subspace containing the three matrices necessarily contains all linear combinations of the three matrices. Conversely, what can we say about the span of the three matrices?

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