A note on the efficiency of position mechanisms with budget constraints

We study the social efficiency of several well-known mechanisms for the allocation of a set of available (advertising) positions to a set of competing budget-constrained users (advertisers). Specifically, we focus on the Generalized Second Price auction (GSP), the Vickrey–Clarke–Groves mechanism (VCG) and the Expressive Generalized First Price auction (EGFP). Using liquid welfare as our efficiency benchmark, we prove a tight bound of 2 on the liquid price of anarchy and stability of these mechanisms for pure Nash equilibria

Hydrologic balance estimation using GIS in Korinthia prefecture, Greece

The main objective of this work is to determine the parameters of hydrological balance for several basins in the prefecture of Korinthia (SE Greece), using hydrometeorological data and geographic information systems (GIS) technology. Multiple linear regression and GIS were used to estimate the spatial distribution of rainfall. The largest precipitation amounts occur in the SW part of the region and decrease towards the eastern and northern coastal parts. The long term mean annual rainfall is 1.39×10<sup>9</sup> m<sup>3</sup>. Based on the Thornthwaite method, infiltration and streamflow were estimated to be 0.29×10<sup>9</sup> m<sup>3</sup>/yr and 0.38×10<sup>9</sup> m<sup>3</sup>/yr, respectively. The unequal distribution of rainfall results in water surplus in the western part of Korinthia prefecture and water deficit in the eastern. We conclude that he estimation of hydrologic balance is a useful tool in order to establish sustainable water resources management in each hydrological basin

Near-optimal Asymmetric Binary Matrix Partitions

We study the asymmetric binary matrix partition problem that was recently introduced by Alon et al. (WINE 2013) to model the impact of asymmetric information on the revenue of the seller in take-it-or-leave-it sales. Instances of the problem consist of an $n \times m$ binary matrix $A$ and a probability distribution over its columns. A partition scheme $B=(B_1,...,B_n)$ consists of a partition $B_i$ for each row $i$ of $A$. The partition $B_i$ acts as a smoothing operator on row $i$ that distributes the expected value of each partition subset proportionally to all its entries. Given a scheme $B$ that induces a smooth matrix $A^B$, the partition value is the expected maximum column entry of $A^B$. The objective is to find a partition scheme such that the resulting partition value is maximized. We present a $9/10$-approximation algorithm for the case where the probability distribution is uniform and a $(1-1/e)$-approximation algorithm for non-uniform distributions, significantly improving results of Alon et al. Although our first algorithm is combinatorial (and very simple), the analysis is based on linear programming and duality arguments. In our second result we exploit a nice relation of the problem to submodular welfare maximization

WATER RESOURCES IN THE WIDER AREA OF THE APOSELEMIS BASIN, CRETE ISLAND, GREECE

Η γνώση του υδρολογικού ισοζυγίου μιας λεκάνης αποτελεί απαραίτητη προϋπόθεση για να εκτιμηθεί η ασφαλής απόδοση των υδροφόρων συστημάτων ώστε να εξασφαλι­ σθεί η ορθολογική και βιώσιμη διαχείριση τους. Προκειμένου να αντιμετωπιστεί η διαταραχή της ισορροπίας του υδατικού ισοζυγίου στην υδρογεωλογική λεκάνη του Αποσελέμη αλλά και στον ευρύτερο χώρο του όρους Δίκτη είναι απαραίτητη η εκτίμη­ ση του υδατικού ισοζυγίου των επιμέρους λεκανών απορροής δηλαδή της λεκάνης του Αποσελέμη, των οροπεδίων Λασιθίου και Καθαρού καθώς και της υπόλοιπης επιφάνειας απορροής του όρους Δίκτη. Προς τούτο χρησιμοποιήθηκαν οι καταγραφές της απορροής του Αποσελέμη στη θέση Ποταμιές και οι μετρήσεις βροχής σε επτά βροχομετρικούς σταθμούς εντός ή πλησίον της λεκάνης απορροής του Αποσελέμη κατά την περίοδο 1967-2003. Τα όρια των περιοχών και οι μορφολογικές παράμετροι καθορί­ στηκαν με τη χρήση GIS, με βάση το ψηφιακό μοντέλο ανάγλυφου (DEM). Η πραγμα­ τική εξατμισοδιαπνοή σε όλη τη περιοχή υπολογίστηκε με τη μέθοδο ThornthwaiteMather. Διαπιστώθηκε υψηλός βαθμός συσχέτισης της απορροής του Αποσελέμη με τη βροχόπτωση στην ευρύτερη περιοχή πέρα από τα συμβατικά όρια της λεκάνης απορ­ ροής του. Σημαντικό ποσοστό της ετήσιας βροχόπτωσης και απορροής στην περιοχή τροφοδοτεί τόσο τα καρστικά υδροφόρα συστήματα κατάντη όσο και την απορροή του Αποσελέμη. Οι καρστικοί υδροφορίες εκφορτίζονται μερικώς από μεγάλες παράκτιες και υποθαλάσσιες υφάλμυρες πηγές, καθώς και από μικρότερες πηγές γλυκού νερού στην ενδοχώρα. Επιβεβαιώθηκε τέλος με ιχνηθετήσεις που εκτέλεσε το ΙΓΜΕ η υ­ δραυλική επικοινωνία του οροπεδίου Λασιθίου και των κατάντη υδροφορέων.The estimation of hydrologie balance is useful in order to assess the safe yield of the aquifer systems and therefore to establish their rational exploitation and sustainable management. Over-abstractions of groundwater in Aposelemis basin resulted in a severe disequilibrium of its water balance. In order to overcome this disequilibrium we studied the hydrological balance in the wider area ofDikti mountain which comprises the drainage basins of Aposelemis river, ofLasithi and Katharo plateaus and of the rest of the drainage area of Dikti. The data used are the monthly values of Aposelemis runoff in the gauge station at Potamies and rainfall data for the period 1967-2003 from seven stations located within or in the close vicinity of the drainage basin of Aposelemis. Using ArcView GIS, the hydrological basin of Aposelemis river and the drainage area ofDikti were delineated, based on the digital elevation model (DEM). The actual évapotranspiration was estimated by the Thornthwaite method. A high inter-relationship exists between the flow of the Aposelemis river and rainfall outside the conventional boundaries of the Aposelemis basin. A significant portion of the annual rainfall, in that wider area is not only recharging the karst systems downstream but also contributing to the flow of Aposelemis river. Karst aquifer systems are partially drained by large coastal and submarine brackish springs and by inland freshwater springs. Finally, tracer studies carried out by IGME confirmed the hydraulic connection between Lasithiplateau and downstream aquifer systems

Adaptive wireless power transfer in mobile ad hoc networks

We investigate the interesting impact of mobility on the problem of efficient wireless power transfer in ad hoc networks. We consider a set of mobile agents (consuming energy to perform certain sensing and communication tasks), and a single static charger (with finite energy) which can recharge the agents when they get in its range. In particular, we focus on the problem of efficiently computing the appropriate range of the charger with the goal of prolonging the network lifetime. We first demonstrate (under the realistic assumption of fixed energy supplies) the limitations of any fixed charging range and, therefore, the need for (and power of) a dynamic selection of the charging range, by adapting to the behavior of the mobile agents which is revealed in an online manner. We investigate the complexity of optimizing the selection of such an adaptive charging range, by showing that two simplified offline optimization problems (closely related to the online one) are NP-hard. To effectively address the involved performance trade-offs, we finally present a variety of adaptive heuristics, assuming different levels of agent information regarding their mobility and energy

Envy-freeness in house allocation problems

We consider the house allocation problem, where m houses are to be assigned to n agents so that each agent gets exactly one house. We present a polynomial-time algorithm that determines whether an envy-free assignment exists, and if so, computes one such assignment. We also show that an envy-free assignment exists with high probability if the number of houses exceeds the number of agents by a logarithmic factor

Aggregating partial rankings with applications to peer grading in massive online open courses

We investigate the potential of using ordinal peer grading for the evaluation of students in massive online open courses (MOOCs). According to such grading schemes, each student receives a few assignments (by other students) which she has to rank. Then, a global ranking (possibly translated into numerical scores) is produced by combining the individual ones. This is a novel application area for social choice concepts and methods where the important problem to be solved is as follows: how should the assignments be distributed so that the collected individual rankings can be easily merged into a global one that is as close as possible to the ranking that represents the relative performance of the students in the assignment? Our main theoretical result suggests that using very simple ways to distribute the assignments so that each student has to rank only k of them, a Borda-like aggregation method can recover a 1 - O(1/k) fraction of the true ranking when each student correctly ranks the assignments she receives. Experimental results strengthen our analysis further and also demonstrate that the same method is extremely robust even when students have imperfect capabilities as graders. Our results provide strong evidence that ordinal peer grading cam be a highly effective and scalable solution for evaluation in MOOCs

Bounding the inefficiency of compromise

Social networks on the Internet have seen an enormous growth recently and play a crucial role in different aspects of today's life. They have facilitated information dissemination in ways that have been beneficial for their users but they are often used strategically in order to spread information that only serves the objectives of particular users. These properties have inspired a revision of classical opinion formation models from sociology using game-theoretic notions and tools. We follow the same modeling approach, focusing on scenarios where the opinion expressed by each user is a compromise between her internal belief and the opinions of a small number of neighbors among her social acquaintances. We formulate simple games that capture this behavior and quantify the inefficiency of equilibria using the well-known notion of the price of anarchy. Our results indicate that compromise comes at a cost that strongly depends on the neighborhood size