315 research outputs found
A mixed integer linear programming heuristics for computing nonstationary (s,S) policy parameters
A Grouping Genetic Algorithm for Joint Stratification and Sample Allocation Designs
Predicting the cheapest sample size for the optimal stratification in
multivariate survey design is a problem in cases where the population frame is
large. A solution exists that iteratively searches for the minimum sample size
necessary to meet accuracy constraints in partitions of atomic strata created
by the Cartesian product of auxiliary variables into larger strata. The optimal
stratification can be found by testing all possible partitions. However the
number of possible partitions grows exponentially with the number of initial
strata. There are alternative ways of modelling this problem, one of the most
natural is using Genetic Algorithms (GA). These evolutionary algorithms use
recombination, mutation and selection to search for optimal solutions. They
often converge on optimal or near-optimal solution more quickly than exact
methods. We propose a new GA approach to this problem using grouping genetic
operators instead of traditional operators. The results show a significant
improvement in solution quality for similar computational effort, corresponding
to large monetary savings.Comment: 22 page
A Neuroevolutionary Approach to Stochastic Inventory Control in Multi-Echelon Systems
Stochastic inventory control in multi-echelon systems poses hard problems in optimisation under uncertainty. Stochastic programming can solve small instances optimally, and approximately solve larger instances via scenario reduction techniques, but it cannot handle arbitrary nonlinear constraints or other non-standard features. Simulation optimisation is an alternative approach that has recently been applied to such problems, using policies that require only a few decision variables to be determined. However, to find optimal or near-optimal solutions we must consider exponentially large scenario trees with a corresponding number of decision variables. We propose instead a neuroevolutionary approach: using an artificial neural network to compactly represent the scenario tree, and training the network by a simulation-based evolutionary algorithm. We show experimentally that this method can quickly find high-quality plans using networks of a very simple form
Generalizing backdoors
Abstract. A powerful intuition in the design of search methods is that one wants to proactively select variables that simplify the problem instance as much as possible when these variables are assigned values. The notion of “Backdoor ” variables follows this intuition. In this work we generalize Backdoors in such a way to allow more general classes of sub-solvers, both complete and heuristic. In order to do so, Pseudo-Backdoors and Heuristic-Backdoors are formally introduced and then applied firstly to a simple Multiple Knapsack Problem and secondly to a complex combinatorial optimization problem in the area of stochastic inventory control. Our preliminary computational experience shows the effectiveness of these approaches that are able to produce very low run times and — in the case of Heuristic-Backdoors — high quality solutions by employing very simple heuristic rules such as greedy local search strategies.
Computing (R, S) policies with correlated demand
This paper considers the single-item single-stocking non-stationary
stochastic lot-sizing problem under correlated demand. By operating under a
nonstationary (R, S) policy, in which R denote the reorder period and S the
associated order-up-to-level, we introduce a mixed integer linear programming
(MILP) model which can be easily implemented by using off-theshelf optimisation
software. Our modelling strategy can tackle a wide range of time-seriesbased
demand processes, such as autoregressive (AR), moving average(MA),
autoregressive moving average(ARMA), and autoregressive with autoregressive
conditional heteroskedasticity process(AR-ARCH). In an extensive computational
study, we compare the performance of our model against the optimal policy
obtained via stochastic dynamic programming. Our results demonstrate that the
optimality gap of our approach averages 2.28% and that computational
performance is good
Exact and approximate solutions for fulfilment metrics in open pipeline order fulfilment systems
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