BUSCADOR RECOLECTA

Web Search Engines store and analyze queries made by their users in order to build their profiles and
offer them personalized search results (i.e., results are ranked according to each user’s preferences). Even
though this service is positive for the users, their profiles may contain sensitive information that can
threaten their privacy. Literature in this field stresses the existence of a trade-off between the level of privacy
achieved and the quality of service received. Current proposals try to maximize this trade-off, yet we
argue that there is still room for improvement. In this paper, we present a new approach which improves
the trade-off by grouping users who share similar profiles. Moreover, our new proposal is based on a
hybrid P2P architecture that outperforms current schemes in terms of scalability. The proposed system
has been tested in terms of performance and privacy achieved, using real data from the AOL dataset. Simulation
results show that: (i) the system protects users’ privacy when they behave honestly, and penalizes
selfish users; (ii) it supports a large number of users; (iii) the runtime required to group users is
affordable; and (iv) groups are dynamic and their topology is unpredictable, which we argue that is positive
from the privacy point of view., This work was partly supported by the Spanish Ministry of Science and Innovation (through Projects eAEGIS TSI2007-65406-C03-01, CO-PRIVACY TIN2011-27076- C03-01, ARES-CONSOLIDER INGENIO 2010 CSD2007-00004, Audit Transparency Voting Process IPT-430000-2010-31 and Ballot Next IPT-2012-0603-430000) and by the Government of Catalonia (under Grant 2009 SGR 1135). Francesc Solsona and Damià Castellà are funded by the MEyC under Contract TIN2011-28689-C02-02. Francesc Solsona is a member of the research group 2009 SGR145, funded by Government of Catalonia.

This paper presents a novel hybrid simulation method based on the combination of an in-house developed 3D ray launching
algorithm and a collaborative filtering (CF) technique, which will be used to analyze the performance of ZigBee-based wireless
sensor networks (WSNs) to enable ambient assisted living (AAL). The combination of Low Definition results obtained by means
of a deterministic ray launching method and the application of a CF technique leads to a drastic reduction of the time and
computational cost required to obtain accurate simulation results. The paper also reports that this kind of AAL indoor complex
scenario withmultiple wireless devices needs a thorough and personalized radioplanning analysis as radiopropagation has a strong
dependence on the network topology and the specific morphology of the scenario. The wireless channel analysis performed by
our hybrid method provides valuable insight into network design phases of complex wireless systems, typical in AAL-oriented
environments.Thus, it results in optimizing network deployment, reducing overall interference levels, and increasing the overall
system performance in terms of cost reduction, transmission rates, and energy efficiency., The authors wish to acknowledge the support provided
under Grant TEC2013-45585-C2-1-R, funded by theMinistry
of Economy and Competitiveness, Spain. URV authors are
partly supported by La Caixa Foundation through Project
SIMPATIC and the Spanish Ministry of Economy and Competitiveness
under Project “Co-Privacy, TIN2011-27076-C03-
01” and by the Government of Catalonia under Grant 2014-
SGR-537.

For planar polynomial vector fields of the form \[ (-y X(x,y)) x (x Y(x,y)) y, \] where X and Y start at least with terms of second order in the variables x and y, we determine necessary and sufficient conditions under which the origin is a center or a uniform isochronous centers.