19 research outputs found
Untangling polygons and graphs
Untangling is a process in which some vertices of a planar graph are moved to
obtain a straight-line plane drawing. The aim is to move as few vertices as
possible. We present an algorithm that untangles the cycle graph C_n while
keeping at least \Omega(n^{2/3}) vertices fixed. For any graph G, we also
present an upper bound on the number of fixed vertices in the worst case. The
bound is a function of the number of vertices, maximum degree and diameter of
G. One of its consequences is the upper bound O((n log n)^{2/3}) for all
3-vertex-connected planar graphs.Comment: 11 pages, 3 figure
On the Obfuscation Complexity of Planar Graphs
Being motivated by John Tantalo's Planarity Game, we consider straight line
plane drawings of a planar graph with edge crossings and wonder how
obfuscated such drawings can be. We define , the obfuscation complexity
of , to be the maximum number of edge crossings in a drawing of .
Relating to the distribution of vertex degrees in , we show an
efficient way of constructing a drawing of with at least edge
crossings. We prove bounds (\delta(G)^2/24-o(1))n^2 < \obf G <3 n^2 for an
-vertex planar graph with minimum vertex degree .
The shift complexity of , denoted by , is the minimum number of
vertex shifts sufficient to eliminate all edge crossings in an arbitrarily
obfuscated drawing of (after shifting a vertex, all incident edges are
supposed to be redrawn correspondingly). If , then
is linear in the number of vertices due to the known fact that the matching
number of is linear. However, in the case we notice that
can be linear even if the matching number is bounded. As for
computational complexity, we show that, given a drawing of a planar graph,
it is NP-hard to find an optimum sequence of shifts making crossing-free.Comment: 12 pages, 1 figure. The proof of Theorem 3 is simplified. An overview
of a related work is adde
A polynomial bound for untangling geometric planar graphs
To untangle a geometric graph means to move some of the vertices so that the
resulting geometric graph has no crossings. Pach and Tardos [Discrete Comput.
Geom., 2002] asked if every n-vertex geometric planar graph can be untangled
while keeping at least n^\epsilon vertices fixed. We answer this question in
the affirmative with \epsilon=1/4. The previous best known bound was
\Omega((\log n / \log\log n)^{1/2}). We also consider untangling geometric
trees. It is known that every n-vertex geometric tree can be untangled while
keeping at least (n/3)^{1/2} vertices fixed, while the best upper bound was
O(n\log n)^{2/3}. We answer a question of Spillner and Wolff [arXiv:0709.0170
2007] by closing this gap for untangling trees. In particular, we show that for
infinitely many values of n, there is an n-vertex geometric tree that cannot be
untangled while keeping more than 3(n^{1/2}-1) vertices fixed. Moreover, we
improve the lower bound to (n/2)^{1/2}.Comment: 14 pages, 7 figure
Planar Drawings of Fixed-Mobile Bigraphs
A fixed-mobile bigraph G is a bipartite graph such that the vertices of one
partition set are given with fixed positions in the plane and the mobile
vertices of the other part, together with the edges, must be added to the
drawing. We assume that G is planar and study the problem of finding, for a
given k >= 0, a planar poly-line drawing of G with at most k bends per edge. In
the most general case, we show NP-hardness. For k=0 and under additional
constraints on the positions of the fixed or mobile vertices, we either prove
that the problem is polynomial-time solvable or prove that it belongs to NP.
Finally, we present a polynomial-time testing algorithm for a certain type of
"layered" 1-bend drawings
Moving Vertices to Make Drawings Plane
A straight-line drawing of a planar graph need not be plane, but
can be made so by moving some of the vertices. Let shift denote the
minimum number of vertices that need to be moved to turn into a plane
drawing of . We show that shift is NP-hard to compute and to
approximate, and we give explicit bounds on shift when is a
tree or a general planar graph. Our hardness results extend to
1BendPointSetEmbeddability, a well-known graph-drawing problem.Comment: This paper has been merged with http://arxiv.org/abs/0709.017