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A polyhedron is defined as the solution set of a finite number of linear equalities and inequalities:

$$P = \{x | a^T_j x ≤ b_j , j = 1, . . . , m, c^T_j x = d_j , j = 1, . . . , p\}$$ A polyhedron is thus the intersection of a finite number of halfspaces and hyper- planes. Affine sets (e.g., subspaces, hyperplanes, lines), rays, line segments, and halfspaces are all polyhedra.

I understand the use of inequalities in the equation. It is used to select the halfspaces included by the polyhedron and thus form a closed structure. But I don't understand:

  • What is the role of the equalities $c^T_j x = d_j , j = 1, . . . , p$

  • And my second question is how is the 'intersection of a finite number of halfspaces and hyper-planes' a polyhedron? i.e for example if we take 2 halfspaces $a^T_1 x ≤ b_1$ and $a^T_2 x \geq b_2$ it will form an unending triangle (informally speaking) covering a sector of the plane. How is this a polyhedron?

NOTE: I am not a mathematician, so use of intuitive terms rather than technical will be more helpful to me.

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    $\begingroup$ Rapidly, for your second question, the polyhedron can be infinite from the definition. It might be surprising at first glance but from a mathematical point of view it is not a problem. $\endgroup$ Apr 6 '20 at 14:14
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    $\begingroup$ The role of the inequalities is to allow polyhedra to have lower dimension than the whole space. For example, a triangle in the plane $\mathbb R^2$ is also a polyhedron in $\mathbb R^3$, and its most natural definition would include the equation $x_3=0$. Of course, any equation $ax=b$ could be replaced by a pair of inequalities $ax\leq b$ and $-ax\leq-b$, so equations aren't absolutely necessary, but it seems more natural to allow them. $\endgroup$ Apr 6 '20 at 15:21
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For your first question, each equality $c_j^T x = d_j$ means that the polyhedron will live inside a particular hyperplane. Intuitively, intersections of multiple hyperplanes can be seen as reducing the dimension of your polyhedron. For instance if $P \subseteq \mathbb{R}^2$ is a $2$-dimensional polyhedron living in the $xy$-plane and we intesect this with the hyperplane $x = 0$ then we are looking at the intersection of $P$ with the $x$-axis. This could be empty, a point or an interval.

For your second question, your example is certainly a polyhedron however not a bounded one. Intuitively the definition of polyhedron is describing which solid objects have 'flat' bounding faces. Usually the important things for a polyhedron are knowing which points lie inside and what its faces look like. Generally it is not a problem if the polyhedron is not bounded.

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