30 research outputs found
Weighted tardiness minimization for unrelated machines with sequence-dependent and resource-constrained setups
Motivated by the need of quick job (re-)scheduling, we examine an elaborate
scheduling environment under the objective of total weighted tardiness
minimization. The examined problem variant moves well beyond existing
literature, as it considers unrelated machines, sequence-dependent and
machine-dependent setup times and a renewable resource constraint on the number
of simultaneous setups. For this variant, we provide a relaxed MILP to
calculate lower bounds, thus estimating a worst-case optimality gap. As a fast
exact approach appears not plausible for instances of practical importance, we
extend known (meta-)heuristics to deal with the problem at hand, coupling them
with a Constraint Programming (CP) component - vital to guarantee the
non-violation of the problem's constraints - which optimally allocates
resources with respect to tardiness minimization. The validity and versatility
of employing different (meta-)heuristics exploiting a relaxed MILP as a quality
measure is revealed by our extensive experimental study, which shows that the
methods deployed have complementary strengths depending on the instance
parameters. Since the problem description has been obtained from a textile
manufacturer where jobs of diverse size arrive continuously under tight
deadlines, we also discuss the practical impact of our approach in terms of
both tardiness decrease and broader managerial insights
One Benders cut to rule all schedules in the neighbourhood
Logic-Based Benders Decomposition (LBBD) and its Branch-and-Cut variant,
namely Branch-and-Check, enjoy an extensive applicability on a broad variety of
problems, including scheduling. Although LBBD offers problem-specific cuts to
impose tighter dual bounds, its application to resource-constrained scheduling
remains less explored. Given a position-based Mixed-Integer Linear Programming
(MILP) formulation for scheduling on unrelated parallel machines, we notice
that certain OPT neighbourhoods could implicitly be explored by regular
local search operators, thus allowing us to integrate Local Branching into
Branch-and-Check schemes. After enumerating such neighbourhoods and obtaining
their local optima - hence, proving that they are suboptimal - a local
branching cut (applied as a Benders cut) eliminates all their solutions at
once, thus avoiding an overload of the master problem with thousands of Benders
cuts. However, to guarantee convergence to optimality, the constructed
neighbourhood should be exhaustively explored, hence this time-consuming
procedure must be accelerated by domination rules or selectively implemented on
nodes which are more likely to reduce the optimality gap. In this study, the
realisation of this idea is limited on the common 'internal (job) swaps' to
construct formulation-specific -OPT neighbourhoods. Nonetheless, the
experimentation on two challenging scheduling problems (i.e., the minimisation
of total completion times and the minimisation of total tardiness on unrelated
machines with sequence-dependent and resource-constrained setups) shows that
the proposed methodology offers considerable reductions of optimality gaps or
faster convergence to optimality. The simplicity of our approach allows its
transferability to other neighbourhoods and different sequencing optimisation
problems, hence providing a promising prospect to improve Branch-and-Check
methods
Surgical Outcomes in Syndromic Tetralogy of Fallot: A Systematic Review and Evidence Quality Assessment
Tetralogy of Fallot (ToF) is one of the most common cyanotic congenital heart defects. We sought to summarize all available data regarding the epidemiology and perioperative outcomes of syndromic ToF patients. A PRISMA-compliant systematic literature review of PubMed and Cochrane Library was performed. Twelve original studies were included. The incidence of syndromic ToF was 15.3% (n = 549/3597). The most prevalent genetic syndromes were 22q11.2 deletion (47.8%; 95% CI 43.4–52.2) and trisomy 21 (41.9%; 95% CI 37.7–46.3). Complete surgical repair was performed in 75.2% of the patients (n = 161/214; 95% CI 69.0–80.1) and staged repair in 24.8% (n = 53/214; 95 CI 19.4–30.9). Relief of RVOT obstruction was performed with transannular patch in 64.7% (n = 79/122; 95% CI 55.9–72.7) of the patients, pulmonary valve-sparing technique in 17.2% (n = 21/122; 95% CI 11.5–24.9), and RV-PA conduit in 18.0% (n = 22/122; 95% CI 12.1–25.9). Pleural effusions were the most common postoperative complications (n = 28/549; 5.1%; 95% CI 3.5–7.3). Reoperations were performed in 4.4% (n = 24/549; 95% CI 2.9–6.4) of the patients. All-cause mortality rate was 9.8% (n = 51/521; 95% CI 7.5–12.7). Genetic syndromes are seen in approximately 15% of ToF patients. Long-term survival exceeds 90%, suggesting that surgical management should be dictated by anatomy regardless of genetics
Αποσύνθεση Logic-Based Benders για προβλήματα μεταφοράς και βιομηχανίας
This thesis examines the ``Logic-Based Benders Decomposition'', a modern partitioning method, originating from the classic Benders Decomposition, which is known since the 1960's. As a large optimisation problem cannot be solved directly by regular exact methods, it is reasonable to partition it into two counterparts, each being more easily solved to optimality. The two counterparts exchange knowledge in an iterative manner, through linear inequalities called ``Benders cuts''. The flexibility of the method allows us to address elaborate problems of large scale, derived from real transportation and manufacturing environments. Motivated by such cases, we show different approaches of the method to provide near-optimal solutions in reasonable time for problems of industrial scale, i.e., for hundreds of orders to manufacture or deliver. Beyond the practical contribution of the thesis, which is the consolidation of efficient LBBD schemes for the problems examined, its technical contribution concerns the construction of generators of valid strong Benders cuts, which can be legitimately utilised on generic classes of optimisation problems. These include the construction of cuts for non-binary integer variables or the elimination of neighbourhoods of multiple solutions with a single inequality without loss of optimality. The applicability of the method, as proved by its successful implementation on the presented problems, occupies most of the structure of the thesis. To reach that, the theoretical framework and the development of the method through the last decades, starting from the fundamentals of Integer Programming and ending at recent relevant literature, is also a major subject in discussion. To conclude, promising extensions, such as the use of the method as a linearisation technique for nonlinear problems or as a mechanism for solving stochastic problems are brought forward to highlight its extendability along with its prospects for future research.Η παρούσα διατριβή εξετάζει την "Αποσύνθεση Logic-Based Benders'', μια σύγχρονη μέθοδο διαχωρισμού, προερχόμενη από την κλασσική Αποσύνθεση Benders. Καθώς ένα μεγάλης κλίμακας πρόβλημα βελτιστοποίησης δεν μπορεί να επιλυθεί απευθείας με τις τυπικές μεθόδους ακριβείας, είναι εύλογος ο διαχωρισμός του σε δύο υπομέρη, καθένα από τα οποία μπορεί να λυθεί βέλτιστα πιο εύκολα. Τα δύο υπομέρη ανταλλάσσουν γνώση επαναληπτικά, χρησιμοποιώντας γραμμικές ανισότητες που ονομάζονται ``Τομές Benders''. Η ευελιξία της μεθόδου μας επιτρέπει να διαχειριζόμαστε περίπλοκα προβλήματα μεγάλης κλίμακας, προερχόμενα από πραγματικά περιβάλλοντα μεταφοράς και βιομηχανίας. Με αφορμή τέτοιες περιπτώσεις, δείχνουμε διαφορετικές προσεγγίσεις της μεθόδου για να υπολογίσουμε σχεδόν βέλτιστες λύσεις σε εύλογο χρόνο για προβλήματα βιομηχανικής κλίμακας, δηλαδή για εκατοντάδες παραγγελίες προς παραγωγή ή παράδοση. Πέρα από την πρακτική συνεισφορά της διατριβής, που αποτελείται από τη συγκέντρωση αποδοτικών σχημάτων LBBD για τα υπό εξέταση προβλήματα, η τεχνική συνεισφορά αφορά τη δημιουργία έγκυρων και ισχυρών τομών Benders, που μπορούν να χρησιμοποιηθούν για γενικευμένες κατηγορίες προβλημάτων βελτιστοποίησης. Οι περιπτώσεις αυτές αφορούν προβλήματα ακέραιων μη δυαδικών μεταβλητών ή την απαλοιφή γειτονιών πολλαπλών λύσεων με χρήση μίας τομής, χωρίς απώλεια της βελτιστότητας. Κυριάρχο θέμα της διατριβής αποτελεί η εφαρμοσιμότητα της μεθόδου, όπως απδεικνύεται από την επιτυχημένη εφαρμογή της στα προβλήματα που παρουσιάζονται. Βασικό θέμα συζήτησης αποτελεί επίσης η περιγραφή του θεωρητικού υποβάθρου και της ανάπτυξης της μεθόδου κατά τις τελευταίες δεκαετίες, ξεκινώντας από τις βασικές αρχές του Ακέραιου Προγραμματισμού και καταλήγοντας στη σύγχρονη συναφή βιβλιογραφία. Τέλος, αναδεικνύονται υποσχόμενες προεκτάσεις, όπως η αξιοποίηση της μεθόδου για τη γραμμικοποίηση μη-γραμμικών προβλημάτων ή ως μηχανισμός επίλυσης στοχαστικών προβλημάτων, ως προοπτικές για μελλοντική έρευνα
A Review of Percutaneous Transluminal Angioplasty in Hemodialysis Fistula
The number of patients in dialysis increases every year. In this review, we will evaluate the role of percutaneous transluminal angioplasty (PTA) according to patency of arteriovenous fistula and grafts. The main indication of PΤΑ is stenosis > 50% or obstruction of the vascular lumen of an arteriovenous fistula and graft. It is usually performed under local anesthesia. The infection rate is as low as the number of complications. Fistula can be used in dialysis in the same day without the need for a central venous catheter. Primary patency is >50% in the first year while primary assisted patency is 80–90% in the same time period. Repeated PTA is as durable as the primary PTA. An early PTA carries a risk of new interventions. Cutting balloon can be used as a second-line method. Stents and covered stents are kept for the management of complications and central outflow venous stenosis. PTA is the treatment of choice for stenosis or obstruction of dialysis fistulas. Repeated PTA may be needed for better patency. Drug eluting balloon may become the future in PTA of dialysis fistula, but more trials are needed