15 votes
Accepted

Is the feasible region of this SDP polyhedral?

No, even if there is a finite number of feasible rank-1 matrices, the feasible region of an SDP does not have to be polyhedral. A spectrahedron you see all the time in applications is $S_n = \{X: X \...
user avatar
12 votes
Accepted

Checking equivalence of two polytopes

I cannot say for sure if you will consider the following approach as better, but from a complexity-theoretic point of view there is a more efficient solution. The idea is to rephrase your question in ...
user avatar
8 votes

Vertices of a polytope

No. Suppose all $a_i$'s are $0$ and all your $b_i$'s are equal; then the polytopes you can get by varying the $b_i$'s are essentially the hypersimplices. But the number of vertices of an $n$-...
user avatar
7 votes
Accepted

Does Horn SAT (Horn formula in CNF) have an integral polytope?

EDIT: Strengthened Theorem 2. The answer to the problem as posed is no, unless P=NP: Theorem 1. Unless P=NP, there is no LP polytope for Horn-SAT that has only integer extreme points and is ...
user avatar
  • 8,273
6 votes
Accepted

Complexity of Finding Largest Set of Intersecting Convex Polytopes

Suppose that the dimension $d$ of the Euclidean space is fixed, and that the input consists of $n$ convex polytopes in $\mathbb{R}^d$ that altogether have $p$ facets. Let $h_1,\ldots,h_p$ denote the ...
user avatar
  • 5,722
6 votes

Reaching the double exponential upper bound in Fourier-Motzkin elimination

I think this upper bound is tight. As an example, consider the following system \begin{align*} +x_0 & +\frac{1}{2} x_1 +\frac{1}{4} x_2 \le 0 \\ +x_0 & +\frac{1}{2} x_1 +\frac{1}{4} x_2 \le ...
user avatar
5 votes

Decide whether a point is a vertex of a polytope?

This answer expands on Chandra's comment, and on my follow up comment. The problem is indeed solvable in polynomial time. More general versions of it are also solvable in polynomial time: $\Theta$ ...
user avatar
5 votes

Checking equivalence of two polytopes

The fact that the underlying polytope $Ax \le b$ is the same for $P_1$ and $P_2$ does not matter, unless we know something specific about $A$ and $b$. This is because a general polytope is an affine ...
user avatar
4 votes
Accepted

Can one efficiently uniformly sample a neighbor of a vertex in the graph of a polytope?

Edit 2: Embarrassingly, there is a two line proof of the $NP$-hardness, if one starts with the right polytope. First, recall the circulation polytope of a graph on the bottom of page 4 of Generating ...
user avatar
  • 1,429
3 votes
Accepted

Properties of convex polytope of 0-1 matrices

Sasho already gave you a yes/no answer, but here's an actual convex combination for you: If $B_\ell$ is the $k \times k$ matrix which is $1$ when $|S_i \cap S_j| \ge \ell$ and zero otherwise, then $M =...
user avatar
2 votes
Accepted

On polytope lattice points

Let's just take the reduction from SAT to IP and see if it works. For a 3-CNF $\phi$, define a polytope $P$ which contains all $x \in \mathbb{R}^n$ satisfying the constraints $0\le x_i \le 1$ for all ...
user avatar
2 votes
Accepted

Reference needed for lower bound on number of guards in three-dimensional art gallery guarding

To my knowledge, Seidel's construction has only been published in O'Rourke's book and nowhere else. In one of his papers, Seidel even refers to O'Rourke's book for a description of his own ...
user avatar
  • 5,722
2 votes
Accepted

When are all facets rank facets? (for independence system polyhedra)

On (1): The case of stable sets in graphs may clarify the situation (a stable set of a graph is a set of pairwise non-adjacent vertices). For each graph $G$, let $\mathcal{S}(G)$ be the set of stable ...
user avatar
  • 36
1 vote
Accepted

Hardness of computing the dimension of an integral polytope?

Yes, MORE-SAT, defined below, is one example of a combinatorial optimization problem for which the natural 0/1 integer linear program (ILP) is guaranteed to be feasible, and determining the dimension ...
user avatar
  • 8,273

Only top scored, non community-wiki answers of a minimum length are eligible