DSpace Collection:
http://hdl.handle.net/10773/317
Tue, 21 Jan 2020 16:52:25 GMT2020-01-21T16:52:25ZInjective edge coloring of graphs
http://hdl.handle.net/10773/27299
Title: Injective edge coloring of graphs
Author: Cardoso, Domingos M.; Cerdeira, J. Orestes; Dominic, Charles; Cruz, J. Pedro
Abstract: Three edges $e_{1}, e_{2}$ and $e_{3}$ in a graph $G$ are consecutive if they form a path (in this order) or a cycle of lengths three. An injective edge coloring of a graph $G = (V,E)$ is a coloring $c$ of the edges of $G$ such that if $e_{1}, e_{2}$ and $e_{3}$ are consecutive edges in $G$, then $c(e_{1})\neq c(e_3)$. The injective edge coloring number $\chi_{i}^{'}(G)$ is the minimum number of colors permitted in such a coloring. In this paper, exact values of $\chi_{i}^{'}(G)$ for several classes of graphs are obtained, upper and lower bounds for $\chi_{i}^{'}(G)$ are introduced and it is proven that checking whether $\chi_{i}^{'}(G)= k$ is NP-complete.Sun, 01 Dec 2019 00:00:00 GMThttp://hdl.handle.net/10773/272992019-12-01T00:00:00ZRegional enlarged observability of Caputo fractional differential equations
http://hdl.handle.net/10773/27243
Title: Regional enlarged observability of Caputo fractional differential equations
Author: Zouiten, Hayat; Boutoulout, Ali; Torres, Delfim F. M.
Abstract: We consider the regional enlarged observability problem for fractional
evolution differential equations involving Caputo derivatives. Using the
Hilbert Uniqueness Method, we show that it is possible to rebuild the initial
state between two prescribed functions only in an internal subregion of the
whole domain. Finally, an example is provided to illustrate the theory.Sun, 01 Mar 2020 00:00:00 GMThttp://hdl.handle.net/10773/272432020-03-01T00:00:00ZSuppliers selection problem with quantity discounts and price changes: a heuristic approach
http://hdl.handle.net/10773/27229
Title: Suppliers selection problem with quantity discounts and price changes: a heuristic approach
Author: Rodrigues, Filipe; Requejo, Cristina
Abstract: This paper addresses a complex suppliers selection problem with multiple products, considering minimum package quantities, minimum order values related to delivery costs and discounted pricing schemes.
Its main contribution is to present an integer linear programming (ILP) model for this suppliers selection problem as well as a model to analyse the impact of prices change. Furthermore, a hybrid heuristic and a genetic algorithm to obtain feasible solutions for this problem are presented.
Several randomly generated examples are solved by using the above two models and the heuristic approaches. Experimental results demonstrate the robustness of the genetic algorithm and allow to realize which are the most important decisions in the suppliers selection problem.Sun, 01 Sep 2019 00:00:00 GMThttp://hdl.handle.net/10773/272292019-09-01T00:00:00ZA computational comparison of compact MILP formulations for the zero forcing number
http://hdl.handle.net/10773/27228
Title: A computational comparison of compact MILP formulations for the zero forcing number
Author: Agra, Agostinho; Cerdeira, Jorge Orestes; Requejo, Cristina
Abstract: Consider a graph where some of its vertices are colored. A colored vertex with a single uncolored neighbor forces that neighbor
to become colored. A zero forcing set is a set of colored vertices that forces all vertices to become colored. The zero forcing number is
the size of a minimum forcing set. Finding the minimum forcing set of a graph is NP-hard.
We give a new compact mixed integer linear programming formulation (MILP) for this problem, and analyse this formulation and establish relation to an existing compact formulation and to two variants. In order to solve large size instances we propose a sequential search algorithm which can also be used as a heuristic to derive upper bounds for the zero forcing number.
A computational study using Xpress (a MILP solver) is conducted to test the performances of the discussed compact formulations and the sequential search algorithm.
We report results on cubic, Watts-Strogatz and randomly generated graphs with 10, 20 and 30 vertices.Mon, 30 Sep 2019 00:00:00 GMThttp://hdl.handle.net/10773/272282019-09-30T00:00:00ZComparing techniques for modelling uncertainty in a maritime inventory routing problem
http://hdl.handle.net/10773/27227
Title: Comparing techniques for modelling uncertainty in a maritime inventory routing problem
Author: Rodrigues, Filipe; Agra, Agostinho; Christiansen, Marielle; Hvattum, Lars Magnus; Requejo, Cristina
Abstract: Uncertainty is inherent in many planning situations. One example is in maritime transportation, where weather conditions and port occupancy are typically characterized by high levels of uncertainty. This paper considers a maritime inventory routing problem where travel times are uncertain. Taking into account possible delays in the travel times is of main importance to avoid inventory surplus or shortages at the storages located at ports.
Several techniques to deal with uncertainty, namely deterministic models with inventory buffers; robust optimization; stochastic programming and models incorporating conditional value-at-risk measures, are considered. The different techniques are tested for their ability to deal with uncertain travel times for a single product maritime inventory routing problem with constant production and consumption rates, a fleet of heterogeneous vessels and multiple ports. At the ports, the product is either produced or consumed and stored in storages with limited capacity. We assume two-stages of decisions, where the routing, the visit order of the ports and the quantities to load/unload are first-stage decisions (fixed before the uncertainty is revealed), while the visit time and the inventory levels at ports are second-stage decisions (adjusted to the observed travel times).
Several solution approaches resulting from the proposed techniques are considered. A computational comparison of the resulting solution approaches is performed to compare the routing costs, the amount of inventory bounds deviation, the total quantities loaded and unloaded, and the running times. This computational experiment is reported for a set of maritime instances having up to six ports and five ships.Thu, 19 Sep 2019 00:00:00 GMThttp://hdl.handle.net/10773/272272019-09-19T00:00:00Z