arXiv subjects

Eli Fox-Epstein

Publications and source records attributed to Eli Fox-Epstein.

5 recordsLinked to original sources

arXiv2015

Diffuse Reflection Radius in a Simple Polygon

It is shown that every simple polygon in general position with $n$ walls can be illuminated from a single point light source $s$ after at most $\lfloor (n-2)/4\rfloor$ diffuse reflections, and this bound is the best possible. A point $s$ with this property can be computed in $O(n\log n)$ time. It is also shown that the minimum number of diffuse reflections needed to illuminate a given simple polygon from a single point can be approximated up to an additive constant in polynomial time.

cs.CG↗

arXiv2015

Diffuse Reflection Diameter in Simple Polygons

We prove a conjecture of Aanjaneya, Bishnu, and Pal that the minimum number of diffuse reflections sufficient to illuminate the interior of any simple polygon with $n$ walls from any interior point light source is $\lfloor n/2 \rfloor - 1$. Light reflecting diffusely leaves a surface in all directions, rather than at an identical angle as with specular reflections.

cs.CG↗

arXiv2015

PTAS for MAP Assignment on Pairwise Markov Random Fields in Planar Graphs

We present a PTAS for computing the maximum a posteriori assignment on Pairwise Markov Random Fields with non-negative weights in planar graphs. This algorithm is practical and not far behind state-of-the-art techniques in image processing. MAP on Pairwise Markov Random Fields with (possibly) negative weights cannot be approximated unless P = NP, even on planar graphs. We also show via reduction that this yields a PTAS for one scoring function of Correlation Clustering in planar graphs.

cs.DM↗

arXiv2014

Linear-Time Algorithm for Sliding Tokens on Trees

Suppose that we are given two independent sets $I_b$ and $I_r$ of a graph such that $|I_b|=|I_r|$, and imagine that a token is placed on each vertex in $I_b$. Then, the sliding token problem is to determine whether there exists a sequence of independent sets which transforms $I_b$ into $I_r$ so that each independent set in the sequence results from the previous one by sliding exactly one token along an edge in the graph. This problem is known to be PSPACE-complete even for planar graphs, and also for bounded treewidth graphs. In this paper, we thus study the problem restricted to trees, and give the following three results: (1) the decision problem is solvable in linear time; (2) for a yes-instance, we can find in quadratic time an actual sequence of independent sets between $I_b$ and $I_r$ whose length (i.e., the number of token-slides) is quadratic; and (3) there exists an infinite family of instances on paths for which any sequence requires quadratic length.

cs.DM↗

arXiv2014

The Convex Configurations of "Sei Shonagon Chie no Ita" and Other Dissection Puzzles

The tangram and Sei Shonagon Chie no Ita are popular dissection puzzles consisting of seven pieces. Each puzzle can be formed by identifying edges from sixteen identical right isosceles triangles. It is known that the tangram can form 13 convex polygons. We show that Sei Shonagon Chie no Ita can form 16 convex polygons, propose a new puzzle that can form 19, no 7 piece puzzle can form 20, and 11 pieces are necessary and sufficient to form all 20 polygons formable by 16 identical isosceles right triangles. Finally, we examine the number of convex polygons formable by different quantities of these triangles.

cs.CG↗