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- Complexity (1)
- Compromise Solutions (1)
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We consider a sorting problem from railway optimization
called train classification: incoming trains are split up into their single
cars and reassembled to form new outgoing trains. Trains are subject
to delay, which may turn a prepared sorting schedule infeasible for the
disturbed situation. The classification methods applied today deal with
this issue by completely disregarding the input order of cars, which provides
robustness against any amount of disturbance but also wastes the
potential contained in the a priori knowledge about the input.
We introduce a new method that provides a feasible sorting schedule for
the expected input and allows to
flexibly insert additional sorting steps
if the schedule has become infeasible after revealing the disturbed input.
By excluding disruptions that almost never occur from our consideration,
we obtain a classification process that is quicker than the current railway
practice but still provides robustness against realistic delays. In fact, our
algorithm allows
flexibly trading off fast classification against high degrees
of robustness depending on the respective need. We further explore
this
flexibility in experiments on real-world traffic data, underlining our
algorithm improves on the methods currently applied in practice.
In this paper we investigate two different recoverable robust models to deal with cost uncertainties in a shortest path problem. Recoverable robustness extends the classical concept of robustness to deal with uncertainties by incorporating limited recovery actions after the
full data are revealed. Our first model focuses on the case where the recovery actions are quite restricted: after a simple path is fixed in the first stage, in the second stage, after all data are revealed, any path containing at most k new arcs may be chosen.
Thus, the parameter k can be interpreted as a mediator between
robust optimization - no changes allowed - and optimization
on the fly - an arbitrary solution can be chosen. Considering three
classical scenario sets, which model uncertainties in the cost function,
we show that this new problem is strongly NP-hard in all
these cases and is not approximable, unless P=NP.
This is in contrast to the robust shortest path problem, where, for
example, an optimal solution can be computed efficiently for interval
and Gamma-scenarios. For series-parallel graphs and interval scenarios,
we present a polynomial time algorithm for this recoverable robust
setting.
In our second model the recovery set, i.e., the set of paths selectable
in the second stage is not limited, but deviating from the previous
choice comes at extra cost. Thus, a path chosen in the first stage
produces renting costs modeled as an alpha-fraction of the scenario
cost. For an arc taken in the second stage the remaining cost needs
to be paid in addition to some extra inflation cost modeled by a beta-fraction
of the scenario cost, if the arc was not reserved beforehand. The
complexity status of this problem is similar to the robust case. Yet,
for Gamma-scenarios the problem is again strongly NP-hard,
but can be approximated.
The knapsack problem is one of the basic problems in combinatorial optimization. In real-world applications it is often part of a more complex problem. Examples are machine capacities in production planning or bandwidth restrictions in telecommunication network design. Due to unpredictable future settings or erroneous data, parameters of such a subproblem are subject to uncertainties.
In high risk situations a robust approach should be chosen to deal with these uncertainties.
Unfortunately, classical robust optimization outputs solutions with little profit by prohibiting any adaption of the solution when the actual realization of the uncertain parameters is known.
This ignores the fact that in most settings minor changes to a previously determined solution are possible. To overcome these drawbacks we allow a limited recovery of a previously fixed item set as soon as the data are known by deleting at most k items and adding up to l new items.
We consider the complexity status of this recoverable robust knapsack problem and extend the classical concept of cover inequalities to obtain stronger polyhedral descriptions. Finally, we present two extensive computational studies to investigate the influence of parameters k and l to the objective and evaluate the effectiveness of our new class of valid inequalities.
In this paper, we investigate the recoverable robust knapsack problem,
where the uncertainty of the item weights follows the approach of Bertsimas and
Sim. In contrast to the robust approach, a limited recovery action is allowed,
i.e., up to k items may be removed when the actual weights are known. This problem
is motivated by the assignment of traffic nodes to antennas in wireless network
planning. Starting from an exponential min-max optimization model, we derive an
integer linear programming formulation of quadratic size. In a preliminary computational
study, we evaluate the gain of recovery using realistic planning data.
We consider a basic subproblem which arises in line planning,
and is of particular importance in the context of a high system
load or robustness: How much can be routed maximally along all possible
lines? The essence of this problem is the Path Constrained Network
Flow (PCN) problem. We explore the complexity of this problem and
its dual. In particular we show for the primal that it is as hard to
approximate as MAX CLIQUE and for the dual that it is as hard to
approximate as SET COVER. We also prove that the PCN problem is
hard for special graph classes, interesting both from a complexity and
from a practical perspective. Finally, we present a special graph class
for which there is a polynomial-time algorithm.
In multicriteria optimization, a compromise solution is a feasible solution whose
cost vector minimizes the distance to the ideal point w.r.t. a given norm. The coor-
dinates of the ideal point are given by the optimal values for the single optimization
problem for each criterion.
We show that the concept of compromise solutions ts nicely into the existing
notion of Pareto optimality: For a huge class of norms, every compromise solution
is Pareto optimal, and under certain conditions on the norm all Pareto optimal so-
lution are also a compromise solution, for an appropriate weighting of the criteria.
Furthermore, under similar conditions on the norm, the existence of an FPTAS for
compromise solutions guarantees the approximability of the Pareto set.
These general results are completed by applications to classical combinatorial
optimization problems. In particular, we study approximation algorithms for the
multicriteria shortest path problem and the multicriteria minimum spanning tree
problem. On the one hand, we derive approximation schemes for both problems, on
the other hand we show that for the latter problem simple approaches like local search
and greedy techniques do not guarantee good approximation factors.