GIVEN
Let $C = \{ x \in \mathbb{R}^n : Ax=b \}$, where $A$ is an $m \times n$ matrix and $b \in \mathbb{R}^m$.
Determine the normal cone $N_C(x)$ and $T_C(x)$ for all $x \in C$.
USEFUL DEFINITIONS
Let $C$ be a nonempty, closed and convex set and let $x \in C$.
Normal Cone
The normal cone of $C$ at $x$ is denoted by $N_C(x)$, and is defined by:
$$z \in N_C(x) \Longleftrightarrow \langle z, c-x\rangle \leq 0, \; \forall c \in C$$
If $x \in \text{int}(C)$ then $N_C(x) = \{ 0 \}$, and if $x \in \text{bdry}(C)$ then then $N_C(x) \neq \{ 0 \}$.($\text{int}$, $\text{cl}$ and $\text{bdry}$ refer to the interior, closure and the boundary).
Tangent Cone
It is defined to be the polar cone of the normal cone.
$$T_C(x) = \big(N_C(x)\big)^\circ = \{ u \in \mathbb{R}^n : \langle u,v \rangle \leq 0, \; \forall v \in N_C(x) \}$$
It can also be expressed as,
$$T_C(x) = \text{cl}\{ \lambda (c-x) : c \in C \text{ and } \lambda \geq 0\}$$
ATTEMPT
I do not clearly understand what $C = \{ x \in \mathbb{R}^n : Ax=b \}$ is, nor do I know how to use its $Ax=b$ property. I am not even sure how to prove that it is closed and convex to use the above definitions.
I first interpreted $Ax=b$ as being a collection of hyperplanes $\langle a^i ,x \rangle = b_i$ with $i=\{1,\ldots,m\}$ and $a^i$ being the $i$th row of $A$. This gives me the impression that $x$ is the intersection of hyperplanes.
I am very confused.
How might I be able to calculate the normal and tangent cones of $C$?
Any help is immensely appreciated.
Best Answer
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