62 research outputs found
Lower Bounds for Shoreline Searching With 2 or More Robots
Searching for a line on the plane with unit speed robots is a classic
online problem that dates back to the 50's, and for which competitive ratio
upper bounds are known for every . In this work we improve the best
lower bound known for robots from 1.5993 to 3. Moreover we prove that the
competitive ratio is at least for robots, and at least
for robots. Our lower bounds match the best upper
bounds known for , hence resolving these cases. To the best of our
knowledge, these are the first lower bounds proven for the cases of
this several decades old problem.Comment: This is an updated version of the paper with the same title which
will appear in the proceedings of the 23rd International Conference on
Principles of Distributed Systems (OPODIS 2019) Neuchatel, Switzerland, July
17-19, 201
Almost-Optimal Deterministic Treasure Hunt in Arbitrary Graphs
A mobile agent navigating along edges of a simple connected graph, either finite or countably infinite, has to find an inert target (treasure) hidden in one of the nodes. This task is known as treasure hunt. The agent has no a priori knowledge of the graph, of the location of the treasure or of the initial distance to it. The cost of a treasure hunt algorithm is the worst-case number of edge traversals performed by the agent until finding the treasure. Awerbuch, Betke, Rivest and Singh [Baruch Awerbuch et al., 1999] considered graph exploration and treasure hunt for finite graphs in a restricted model where the agent has a fuel tank that can be replenished only at the starting node s. The size of the tank is B = 2(1+?)r, for some positive real constant ?, where r, called the radius of the graph, is the maximum distance from s to any other node. The tank of size B allows the agent to make at most {? B?} edge traversals between two consecutive visits at node s.
Let e(d) be the number of edges whose at least one extremity is at distance less than d from s. Awerbuch, Betke, Rivest and Singh [Baruch Awerbuch et al., 1999] conjectured that it is impossible to find a treasure hidden in a node at distance at most d at cost nearly linear in e(d). We first design a deterministic treasure hunt algorithm working in the model without any restrictions on the moves of the agent at cost ?(e(d) log d), and then show how to modify this algorithm to work in the model from [Baruch Awerbuch et al., 1999] with the same complexity. Thus we refute the above twenty-year-old conjecture. We observe that no treasure hunt algorithm can beat cost ?(e(d)) for all graphs and thus our algorithms are also almost optimal
Almost-Optimal Deterministic Treasure Hunt in Arbitrary Graphs
A mobile agent navigating along edges of a simple connected graph, either
finite or countably infinite, has to find an inert target (treasure) hidden in
one of the nodes. This task is known as treasure hunt. The agent has no a
priori knowledge of the graph, of the location of the treasure or of the
initial distance to it. The cost of a treasure hunt algorithm is the worst-case
number of edge traversals performed by the agent until finding the treasure.
Awerbuch, Betke, Rivest and Singh [3] considered graph exploration and treasure
hunt for finite graphs in a restricted model where the agent has a fuel tank
that can be replenished only at the starting node . The size of the tank is
, for some positive real constant , where , called
the radius of the graph, is the maximum distance from to any other node.
The tank of size allows the agent to make at most edge
traversals between two consecutive visits at node .
Let be the number of edges whose at least one extremity is at distance
less than from . Awerbuch, Betke, Rivest and Singh [3] conjectured that
it is impossible to find a treasure hidden in a node at distance at most at
cost nearly linear in . We first design a deterministic treasure hunt
algorithm working in the model without any restrictions on the moves of the
agent at cost , and then show how to modify this
algorithm to work in the model from [3] with the same complexity. Thus we
refute the above twenty-year-old conjecture. We observe that no treasure hunt
algorithm can beat cost for all graphs and thus our algorithms
are also almost optimal
- …