60,324 research outputs found
A Dual Ascent Procedure for Large Scale Uncapacitated Network Design
The fixed-charge network design problem arises in a variety of problem contexts including transportation, communication, and production scheduling.We develop a family of dual ascent algorithms for this problem. This approach generalizes known ascent procedures for solving shortest path, plant location,Steiner network and directed spanning tree problems. Our computational results for several classes of test problems with up to 500 integer and 1.98 million continuous variables and constraints shows that the dual ascent procedure and an associated drop-add heuristic generates solutions that, in almost all cases, are guaranteed to be within 1 to 3 percent of optimality. Moreover, the procedure requires no more than 150 seconds on an IBM 3083 computer. The test problems correspond to dense and sparse networks,including some models arising in freight transport
Computing shortest paths in 2D and 3D memristive networks
Global optimisation problems in networks often require shortest path length
computations to determine the most efficient route. The simplest and most
common problem with a shortest path solution is perhaps that of a traditional
labyrinth or maze with a single entrance and exit. Many techniques and
algorithms have been derived to solve mazes, which often tend to be
computationally demanding, especially as the size of maze and number of paths
increase. In addition, they are not suitable for performing multiple shortest
path computations in mazes with multiple entrance and exit points. Mazes have
been proposed to be solved using memristive networks and in this paper we
extend the idea to show how networks of memristive elements can be utilised to
solve multiple shortest paths in a single network. We also show simulations
using memristive circuit elements that demonstrate shortest path computations
in both 2D and 3D networks, which could have potential applications in various
fields
Semi-classical Dynamical Triangulations
For non-critical string theory the partition function reduces to an integral
over moduli space after integrating over matter fields. The moduli integrand is
known analytically for genus one surfaces. The formalism of dynamical
triangulations provides us with a regularization of non-critical string theory
and we show that even for very small triangulations it reproduces very well the
continuum integrand when the central charge of the matter fields is large
negative, thus providing a striking example of how the quantum fluctuations of
geometry disappear when .Comment: 11 pages, 5 figure
Improved Algorithms for Decremental Single-Source Reachability on Directed Graphs
Recently we presented the first algorithm for maintaining the set of nodes
reachable from a source node in a directed graph that is modified by edge
deletions with total update time, where is the number of edges and
is the number of nodes in the graph [Henzinger et al. STOC 2014]. The
algorithm is a combination of several different algorithms, each for a
different vs. trade-off. For the case of the
running time is , just barely below . In
this paper we simplify the previous algorithm using new algorithmic ideas and
achieve an improved running time of . This gives,
e.g., for the notorious case . We obtain the
same upper bounds for the problem of maintaining the strongly connected
components of a directed graph undergoing edge deletions. Our algorithms are
correct with high probabililty against an oblivious adversary.Comment: This paper was presented at the International Colloquium on Automata,
Languages and Programming (ICALP) 2015. A full version combining the findings
of this paper and its predecessor [Henzinger et al. STOC 2014] is available
at arXiv:1504.0795
Polynomial cases of the tarification problem
We consider the problem of determining a set of optimal tariffs for an agent in a network, who owns a subset of the arcs of the network, and who wishes to maximize his revenues on this subset from a set of clients that make use of the network.The general variant of this problem is NP-hard, already with a single client. This paper introduces several new polynomially solvable special cases. An important case is the following.For multiple clients, if the number of tariff arcs is bounded from above, we can solve the problem by a polynomial number of linear programs (each of which is of polynomial size). Furthermore, we show that the parametric tarification problem and the single arc fixed charge tarification problem can be solved in polynomial time.Economics ;
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