27 research outputs found
Roundtrip Spanners with (2k-1) Stretch
A roundtrip spanner of a directed graph is a subgraph of preserving
roundtrip distances approximately for all pairs of vertices. Despite extensive
research, there is still a small stretch gap between roundtrip spanners in
directed graphs and undirected graphs. For a directed graph with real edge
weights in , we first propose a new deterministic algorithm that
constructs a roundtrip spanner with stretch and edges for every integer , then remove the dependence of size on
to give a roundtrip spanner with stretch and edges. While keeping the edge size small, our result improves the previous
stretch roundtrip spanners in directed graphs [Roditty, Thorup,
Zwick'02; Zhu, Lam'18], and almost matches the undirected -spanner with
edges [Alth\"ofer et al. '93] when is a constant, which is
optimal under Erd\"os conjecture.Comment: 12 page
Improved Roundtrip Spanners, Emulators, and Directed Girth Approximation
Roundtrip spanners are the analog of spanners in directed graphs, where the
roundtrip metric is used as a notion of distance. Recent works have shown
existential results of roundtrip spanners nearly matching the undirected case,
but the time complexity for constructing roundtrip spanners is still widely
open.
This paper focuses on developing fast algorithms for roundtrip spanners and
related problems. For any -vertex directed graph with edges (with
non-negative edge weights), our results are as follows:
- 3-roundtrip spanner faster than APSP: We give an
-time algorithm that constructs a roundtrip spanner of
stretch and optimal size . Previous constructions of roundtrip
spanners of the same size either required time [Roditty, Thorup,
Zwick SODA'02; Cen, Duan, Gu ICALP'20], or had worse stretch [Chechik and
Lifshitz SODA'21].
- Optimal roundtrip emulator in dense graphs: For integer , we give
an -time algorithm that constructs a roundtrip \emph{emulator}
of stretch and size , which is optimal for constant
under Erd\H{o}s' girth conjecture. Previous work of [Thorup and Zwick STOC'01]
implied a roundtrip emulator of the same size and stretch, but it required
construction time. Our improved running time is near-optimal for
dense graphs.
- Faster girth approximation in sparse graphs: We give an
-time algorithm that -approximates the girth of a
directed graph. This can be compared with the previous -approximation
algorithm in time by [Chechik and Lifshitz
SODA'21]. In sparse graphs, our algorithm achieves better running time at the
cost of a larger approximation ratio.Comment: To appear in SODA 202
A Fast Algorithm for Source-Wise Round-Trip Spanners
In this paper, we study the problem of efficiently constructing source-wise
round-trip spanners in weighted directed graphs. For a source vertex set
in a digraph , an -source-wise round-trip spanner of
of stretch is a subgraph of such that for every , the round-trip distance between and in is at most times of
the original distance in . We show that, for a digraph with
vertices, edges and nonnegative edge weights, an -sized source vertex
set and a positive integer , there exists an algorithm, in
time , with high probability constructing an
-source-wise round-trip spanner of stretch and size
. Compared with the state of the art for constructing
source-wise round-trip spanners, our algorithm significantly improves their
construction time (where
and 2.373 is the matrix multiplication exponent) to nearly linear
, while still keeping a spanner stretch and
size , asymptotically similar to their stretch
and size , respectively
Relaxed spanners for directed disk graphs
Let be a finite metric space, where is a set of points
and is a distance function defined for these points. Assume that
has a constant doubling dimension and assume that each point
has a disk of radius around it. The disk graph that corresponds
to and is a \emph{directed} graph , whose vertices are
the points of and whose edge set includes a directed edge from to
if . In \cite{PeRo08} we presented an algorithm for
constructing a (1+\eps)-spanner of size O(n/\eps^d \log M), where is
the maximal radius . The current paper presents two results. The first
shows that the spanner of \cite{PeRo08} is essentially optimal, i.e., for
metrics of constant doubling dimension it is not possible to guarantee a
spanner whose size is independent of . The second result shows that by
slightly relaxing the requirements and allowing a small perturbation of the
radius assignment, considerably better spanners can be constructed. In
particular, we show that if it is allowed to use edges of the disk graph
I(V,E,r_{1+\eps}), where r_{1+\eps}(p) = (1+\eps)\cdot r(p) for every , then it is possible to get a (1+\eps)-spanner of size O(n/\eps^d) for
. Our algorithm is simple and can be implemented efficiently
Spanning Properties of Theta-Theta Graphs
We study the spanning properties of Theta-Theta graphs. Similar in spirit
with the Yao-Yao graphs, Theta-Theta graphs partition the space around each
vertex into a set of k cones, for some fixed integer k > 1, and select at most
one edge per cone. The difference is in the way edges are selected. Yao-Yao
graphs select an edge of minimum length, whereas Theta-Theta graphs select an
edge of minimum orthogonal projection onto the cone bisector. It has been
established that the Yao-Yao graphs with parameter k = 6k' have spanning ratio
11.67, for k' >= 6. In this paper we establish a first spanning ratio of
for Theta-Theta graphs, for the same values of . We also extend the class of
Theta-Theta spanners with parameter 6k', and establish a spanning ratio of
for k' >= 5. We surmise that these stronger results are mainly due to a
tighter analysis in this paper, rather than Theta-Theta being superior to
Yao-Yao as a spanner. We also show that the spanning ratio of Theta-Theta
graphs decreases to 4.64 as k' increases to 8. These are the first results on
the spanning properties of Theta-Theta graphs.Comment: 20 pages, 6 figures, 3 table
Fully Dynamic Spanners with Worst-Case Update Time
An alpha-spanner of a graph G is a subgraph H such that H preserves all distances of G within a factor of alpha. In this paper, we give fully dynamic algorithms for maintaining a spanner H of a graph G undergoing edge insertions and deletions with worst-case guarantees on the running time after each update. In particular, our algorithms maintain:
- a 3-spanner with ~O(n^{1+1/2}) edges with worst-case update time ~O(n^{3/4}), or
- a 5-spanner with ~O(n^{1+1/3}) edges with worst-case update time ~O (n^{5/9}).
These size/stretch tradeoffs are best possible (up to logarithmic factors). They can be extended to the weighted setting at very minor cost. Our algorithms are randomized and correct with high probability against an oblivious adversary. We also further extend our techniques to construct a 5-spanner with suboptimal size/stretch tradeoff, but improved worst-case update time.
To the best of our knowledge, these are the first dynamic spanner algorithms with sublinear worst-case update time guarantees. Since it is known how to maintain a spanner using small amortized}but large worst-case update time [Baswana et al. SODA\u2708], obtaining algorithms with strong worst-case bounds, as presented in this paper, seems to be the next natural step for this problem
Parameterized Complexity of Directed Spanner Problems
We initiate the parameterized complexity study of minimum t-spanner problems on directed graphs. For a positive integer t, a multiplicative t-spanner of a (directed) graph G is a spanning subgraph H such that the distance between any two vertices in H is at most t times the distance between these vertices in G, that is, H keeps the distances in G up to the distortion (or stretch) factor t. An additive t-spanner is defined as a spanning subgraph that keeps the distances up to the additive distortion parameter t, that is, the distances in H and G differ by at most t. The task of Directed Multiplicative Spanner is, given a directed graph G with m arcs and positive integers t and k, decide whether G has a multiplicative t-spanner with at most arcs. Similarly, Directed Additive Spanner asks whether G has an additive t-spanner with at most arcs. We show that (i) Directed Multiplicative Spanner admits a polynomial kernel of size and can be solved in randomized time, (ii) the weighted variant of Directed Multiplicative Spanner can be solved in time on directed acyclic graphs, (iii) Directed Additive Spanner is -hard when parameterized by k for every fixed even when the input graphs are restricted to be directed acyclic graphs. The latter claim contrasts with the recent result of Kobayashi from STACS 2020 that the problem for undirected graphs is when parameterized by t and k.publishedVersio