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a.) Show that if $A=A^T$ is a symmetric matrix, then $A\mathbf{x}=\mathbf{b}$ has a solution iff b is orthogonal to $\ker A$.

b.) Prove that if $K$ is a positive semi-definite matrix and $\mathbf{f}\notin \operatorname{rng}K$, then the quadratic function $$p(\mathbf{x}) = \mathbf{x}^\mathrm{T}K\mathbf{x} -2\mathbf{x}^\mathrm{T}\mathbf{f} + c$$ has no minimum value.

c.) Suppose $\{\mathbf{v_1},\ \cdots,\ \mathbf{v_n}\}$ span a subspace $V \subset \mathbb{R}^m$. Prove that $\mathbf{w}$ is orthogonal to $V$ iff $\mathbf{w}\in \operatorname{coker}A$ where $A=\begin{pmatrix}\mathbf{v_1} & \mathbf{v_2} & \cdots & \mathbf{v_n}\end{pmatrix}$ is the matrix with the indicated columns.

My attempt:

a.) We know that $A=A^\mathrm{T}$, then a vector $\mathbf{x}\in \mathbb{R}^n$ lies in $\ker A$ iff $A\mathbf{x} = \mathbf{0}$.

By matrix multiplication we know that the $i^{\text{th}}$ entry of $A\mathbf{x}$ equals the vector product of the $i^{\text{th}}$ row $\mathbf{r_i}^T$ of $A$ and $\mathbf{x}$, hence $\mathbf{r_i}^{T}\cdot \mathbf{x} = \mathbf{r_i} \cdot \mathbf{x} = 0$ iff $\mathbf{x}$ is orthogonal to $\mathbf{r_i}$.

Therefore $\mathbf{x}\in \ker A$ iff $\mathbf{x}$ is orthogonal to all the rows of $A$. Thus $A\mathbf{x} = \mathbf{b}$ has a solution iff $\mathbf{b}$ is orthogonal to $\ker A$. Is this correct?

b.) I do not know how to do this

c.) In this do I have to prove that $\mathbf{w} \in \operatorname{coker}A$ is orthogonal to the range of $A$? I am not exactly sure what they are asking.

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Part b seems a bit weird. If $\mathbf{f} \notin \operatorname{rng}(K)$ then there is no minimum. Otherwise, there is a global minimum, just that it's not unique. – EuYu Nov 18 '12 at 2:03
@EuYu oops, you are right. I forgot to put the not equal sign. Thanks for catching that! – diimension Nov 18 '12 at 2:10
up vote 1 down vote accepted

For a, your argument is showing that the rowspace is the orthogonal complement of the nullspace. That is not what the question is asking (although it is related). Notice that $\mathbf{b}$ is an element in the columnspace of $A$. The columnspace, rowspace and nullspace of a symmetric matrix are related in a very specific manner. See if you can use these relations.

For part b, consider the critical points of the quadratic form. Where do they occur? Does your function have any?

For c, what can you say about the left-nullspace and the columnspace of a matrix?

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Thanks for the advice! For a.) and c.) b is an element of the columnspace so I have to show that the columnspace is orthogonal to the left-nullspace? For b.) they occur when it is minimized but how can I find that? – diimension Nov 18 '12 at 3:56
Well, for a symmetric matrix, the columnspace is the rowspace. – EuYu Nov 18 '12 at 3:57
There is where I am confused because $A^T=A$ so then I just have to show the rowspace is orthogonal? – diimension Nov 18 '12 at 4:00
$A^\mathrm{T} = A$ means that the rows and the columns of the matrix coincide. So of course the rowspace and the columnspace of the matrix coincide. Your vector $\mathbf{b}$ is a vector in the columnspace of $A$ and hence it's a vector in the rowspace of $A$. For part b, littleO's solution is perhaps more natural but taking critical points is certainly easier to think of. The gradient of a quadratic form takes on a very specific form. See this link for example. – EuYu Nov 18 '12 at 4:02
So I just had to clarify in my proof that "the rowspace and the columnspace of the matrix coincide where vector b is a vector in the columnspace of A and hence it's a vector in the rowspace of A" then it will be suffice ? And thanks for the link I am going to take a look at it now! – diimension Nov 18 '12 at 4:06

I'll assume $A$ has real entries.

For part a), \begin{align*} & u \in R(A)^{\perp} \\ \iff & \langle u, y \rangle = 0 \, \forall \, y \in R(A) \\ \iff & \langle u, Ax \rangle = 0 \, \forall \, x \\ \iff & \langle A^T u, x \rangle = 0 \, \forall \, x \\ \iff & A^T u = 0 \\ \iff & u \in N(A^T). \end{align*} This shows that $N(A^T)$ is the orthogonal complement of $R(A)$. Because $A$ is symmetric, $N(A)$ is the orthogonal complement of $R(A)$. Hence $b \in R(A)$ if and only if $b$ is orthogonal to $N(A)$.

For part b), let $v$ be the projection of $f$ onto $N(A)$. Note that $v^T f = \|v\|^2 \neq 0$ and the function \begin{align*} g(\alpha) &= p(\alpha v) \\ &= -2\alpha v^T f + c \end{align*} is unbounded below.

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Thanks! But I don't understand the notation you are using in part a. Do you mind if you can explain the proof by words? – diimension Nov 18 '12 at 4:04
I think I have a proof for a). Let $\mathbf t \in \text{ker}A$ and $\mathbf x$ such that $A\mathbf x=\mathbf b$. A linear operator $A$ is symmetric, with respect of $\langle \cdot,\cdot\rangle$, iff $ \langle A\mathbf v, \mathbf w\rangle = \langle\mathbf v,A\mathbf w\rangle\ \forall \mathbf v, \mathbf w \in V $ If you substitute $\mathbf v=\mathbf x$ and $\mathbf w=\mathbf t$ you get $ \langle A\mathbf x, \mathbf t\rangle = \langle \mathbf x, A\mathbf t\rangle \Rightarrow \langle\mathbf b, t\rangle = \langle\mathbf x, \underline O\rangle=0 $ So $\mathbf b$ is orthogonal to $\text{ker}A$. – Rnhmjoj Jan 19 at 16:56

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