401 research outputs found
On the fine-grained complexity of rainbow coloring
The Rainbow k-Coloring problem asks whether the edges of a given graph can be
colored in colors so that every pair of vertices is connected by a rainbow
path, i.e., a path with all edges of different colors. Our main result states
that for any , there is no algorithm for Rainbow k-Coloring running in
time , unless ETH fails.
Motivated by this negative result we consider two parameterized variants of
the problem. In Subset Rainbow k-Coloring problem, introduced by Chakraborty et
al. [STACS 2009, J. Comb. Opt. 2009], we are additionally given a set of
pairs of vertices and we ask if there is a coloring in which all the pairs in
are connected by rainbow paths. We show that Subset Rainbow k-Coloring is
FPT when parameterized by . We also study Maximum Rainbow k-Coloring
problem, where we are additionally given an integer and we ask if there is
a coloring in which at least anti-edges are connected by rainbow paths. We
show that the problem is FPT when parameterized by and has a kernel of size
for every (thus proving that the problem is FPT), extending the
result of Ananth et al. [FSTTCS 2011]
Fine-Grained Complexity of Rainbow Coloring and its Variants
Consider a graph G and an edge-coloring c_R:E(G) rightarrow [k]. A rainbow path between u,v in V(G) is a path P from u to v such that for all e,e\u27 in E(P), where e neq e\u27 we have c_R(e) neq c_R(e\u27). In the Rainbow k-Coloring problem we are given a graph G, and the objective is to decide if there exists c_R: E(G) rightarrow [k] such that for all u,v in V(G) there is a rainbow path between u and v in G. Several variants of Rainbow k-Coloring have been studied, two of which are defined as follows. The Subset Rainbow k-Coloring takes as an input a graph G and a set S subseteq V(G) times V(G), and the objective is to decide if there exists c_R: E(G) rightarrow [k] such that for all (u,v) in S there is a rainbow path between u and v in G. The problem Steiner Rainbow k-Coloring takes as an input a graph G and a set S subseteq V(G), and the objective is to decide if there exists c_R: E(G) rightarrow [k] such that for all u,v in S there is a rainbow path between u and v in G. In an attempt to resolve open problems posed by Kowalik et al. (ESA 2016), we obtain the following results.
- For every k geq 3, Rainbow k-Coloring does not admit an algorithm running in time 2^{o(|E(G)|)}n^{O(1)}, unless ETH fails.
- For every k geq 3, Steiner Rainbow k-Coloring does not admit an algorithm running in time 2^{o(|S|^2)}n^{O(1)}, unless ETH fails.
- Subset Rainbow k-Coloring admits an algorithm running in time 2^{OO(|S|)}n^{O(1)}. This also implies an algorithm running in time 2^{o(|S|^2)}n^{O(1)} for Steiner Rainbow k-Coloring, which matches the lower bound we obtain
Algorithms and Bounds for Very Strong Rainbow Coloring
A well-studied coloring problem is to assign colors to the edges of a graph
so that, for every pair of vertices, all edges of at least one shortest
path between them receive different colors. The minimum number of colors
necessary in such a coloring is the strong rainbow connection number
(\src(G)) of the graph. When proving upper bounds on \src(G), it is natural
to prove that a coloring exists where, for \emph{every} shortest path between
every pair of vertices in the graph, all edges of the path receive different
colors. Therefore, we introduce and formally define this more restricted edge
coloring number, which we call \emph{very strong rainbow connection number}
(\vsrc(G)).
In this paper, we give upper bounds on \vsrc(G) for several graph classes,
some of which are tight. These immediately imply new upper bounds on \src(G)
for these classes, showing that the study of \vsrc(G) enables meaningful
progress on bounding \src(G). Then we study the complexity of the problem to
compute \vsrc(G), particularly for graphs of bounded treewidth, and show this
is an interesting problem in its own right. We prove that \vsrc(G) can be
computed in polynomial time on cactus graphs; in contrast, this question is
still open for \src(G). We also observe that deciding whether \vsrc(G) = k
is fixed-parameter tractable in and the treewidth of . Finally, on
general graphs, we prove that there is no polynomial-time algorithm to decide
whether \vsrc(G) \leq 3 nor to approximate \vsrc(G) within a factor
, unless PNP
Open questions about Ramsey-type statements in reverse mathematics
Ramsey's theorem states that for any coloring of the n-element subsets of N
with finitely many colors, there is an infinite set H such that all n-element
subsets of H have the same color. The strength of consequences of Ramsey's
theorem has been extensively studied in reverse mathematics and under various
reducibilities, namely, computable reducibility and uniform reducibility. Our
understanding of the combinatorics of Ramsey's theorem and its consequences has
been greatly improved over the past decades. In this paper, we state some
questions which naturally arose during this study. The inability to answer
those questions reveals some gaps in our understanding of the combinatorics of
Ramsey's theorem.Comment: 15 page
Complexity of Computing the Anti-Ramsey Numbers for Paths
The anti-Ramsey numbers are a fundamental notion in graph theory, introduced
in 1978, by Erd\" os, Simonovits and S\' os. For given graphs and the
\emph{anti-Ramsey number} is defined to be the maximum
number such that there exists an assignment of colors to the edges of
in which every copy of in has at least two edges with the same
color.
There are works on the computational complexity of the problem when is a
star. Along this line of research, we study the complexity of computing the
anti-Ramsey number , where is a path of length .
First, we observe that when , the problem is hard; hence, the
challenging part is the computational complexity of the problem when is a
fixed constant.
We provide a characterization of the problem for paths of constant length.
Our first main contribution is to prove that computing for
every integer is NP-hard. We obtain this by providing several structural
properties of such coloring in graphs. We investigate further and show that
approximating to a factor of is hard
already in -partite graphs, unless P=NP. We also study the exact complexity
of the precolored version and show that there is no subexponential algorithm
for the problem unless ETH fails for any fixed constant .
Given the hardness of approximation and parametrization of the problem, it is
natural to study the problem on restricted graph families. We introduce the
notion of color connected coloring and employing this structural property. We
obtain a linear time algorithm to compute , for every
integer , when the host graph, , is a tree
Chasing the Rainbow Connection: Hardness, Algorithms, and Bounds
We study rainbow connectivity of graphs from the algorithmic and graph-theoretic points of view. The study is divided into three parts. First, we study the complexity of deciding whether a given edge-colored graph is rainbow-connected. That is, we seek to verify whether the graph has a path on which no color repeats between each pair of its vertices. We obtain a comprehensive map of the hardness landscape of the problem. While the problem is NP-complete in general, we identify several structural properties that render the problem tractable. At the same time, we strengthen the known NP-completeness results for the problem. We pinpoint various parameters for which the problem is ïŹxed-parameter tractable, including dichotomy results for popular width parameters, such as treewidth and pathwidth. The study extends to variants of the problem that consider vertex-colored graphs and/or rainbow shortest paths. We also consider upper and lower bounds for exact parameterized algorithms. In particular, we show that when parameterized by the number of colors k, the existence of a rainbow s-t path can be decided in Oâ (2k ) time and polynomial space. For the highly related problem of ïŹnding a path on which all the k colors appear, i.e., a colorful path, we explain the modest progress over the last twenty years. Namely, we prove that the existence of an algorithm for ïŹnding a colorful path in (2 â Δ)k nO(1) time for some Δ > 0 disproves the so-called Set Cover Conjecture.Second, we focus on the problem of ïŹnding a rainbow coloring. The minimum number of colors for which a graph G is rainbow-connected is known as its rainbow connection number, denoted by rc(G). Likewise, the minimum number of colors required to establish a rainbow shortest path between each pair of vertices in G is known as its strong rainbow connection number, denoted by src(G). We give new hardness results for computing rc(G) and src(G), including their vertex variants. The hardness results exclude polynomial-time algorithms for restricted graph classes and also fast exact exponential-time algorithms (under reasonable complexity assumptions). For positive results, we show that rainbow coloring is tractable for e.g., graphs of bounded treewidth. In addition, we give positive parameterized results for certain variants and relaxations of the problems in which the goal is to save k colors from a trivial upper bound, or to rainbow connect only a certain number of vertex pairs.Third, we take a more graph-theoretic view on rainbow coloring. We observe upper bounds on the rainbow connection numbers in terms of other well-known graph parameters. Furthermore, despite the interest, there have been few results on the strong rainbow connection number of a graph. We give improved bounds and determine exactly the rainbow and strong rainbow connection numbers for some subclasses of chordal graphs. Finally, we pose open problems and conjectures arising from our work
TextureGAN: Controlling Deep Image Synthesis with Texture Patches
In this paper, we investigate deep image synthesis guided by sketch, color,
and texture. Previous image synthesis methods can be controlled by sketch and
color strokes but we are the first to examine texture control. We allow a user
to place a texture patch on a sketch at arbitrary locations and scales to
control the desired output texture. Our generative network learns to synthesize
objects consistent with these texture suggestions. To achieve this, we develop
a local texture loss in addition to adversarial and content loss to train the
generative network. We conduct experiments using sketches generated from real
images and textures sampled from a separate texture database and results show
that our proposed algorithm is able to generate plausible images that are
faithful to user controls. Ablation studies show that our proposed pipeline can
generate more realistic images than adapting existing methods directly.Comment: CVPR 2018 spotligh
Manual / Issue 10 / Polychrome
Manual, a journal about art and its making. Polychrome. In art, especially, polychrome invites us to the dialogue that colors are always having amongst themselves. A history of polychrome could be a series of poems exchanged among colors. The exchange might exhibit something like perpetual newness, again and again revealing differently bent hues and movingly novel blends. It would be a short-line poetry, excruciatingly sensitive to tone. Its speakers would have no names, so it would confuse the psychology of human orientation. In this connection, a warning against rendering polychrome as a pure positive seems in order: the parties to this dialogue talk at cross-purposes, always on the brink of divorcing. Polychrome can offend and destroy. It conscripts discrete colors in order to sacrifice them. Does polychrome offend by mocking our own failure to connect? In any case, polychrome has an advanced idiom for dealing with conflict. Itâs at home with uncertainty. âDarby English, from the introduction to Issue 10: Polychrome.
Softcover, 80 pages. Published 2018 by the RISD Museum. Manual 10 (Polychrome) contributors include David Batchelor, Gina Borromeo, Nicole Buchanan, Catherine Cooper, Darby English, Mara L. Hermano, Elon Cook Lee, Josephine Lee, Evelyn Lincoln, Dominic Molon, Maureen C. O\u27Brien, RISD Museum 2017 Summer Teen Intensive Students, and Elizabeth A. Williams.https://digitalcommons.risd.edu/risdmuseum_journals/1036/thumbnail.jp
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