1,721,011 research outputs found
Computer-aided proof of Erdős discrepancy properties
Full text linkIn 1930s Paul Erdős conjectured that for any positive integer C in any infinite ±1 sequence (xn) there exists a subsequence xd, x2d, x3d, ..., xkd, for some positive integers k and d, such that |∑ki=1xi·d| >C. The conjecture has been referred to as one of the major open problems in combinatorial number theory and discrepancy theory. For the particular case of C = 1 a human proof of the conjecture exists; for C = 2 a bespoke computer program had generated sequences of length 1124 of discrepancy 2, but the status of the conjecture remained open even for such a small bound. We show that by encoding the problem into Boolean satisfiability and applying the state of the art SAT solvers, one can obtain a discrepancy 2 sequence of length 1160 and a proof of the Erdős discrepancy conjecture for C = 2, claiming that no discrepancy 2 sequence of length 1161, or more, exists. In the similar way, we obtain a precise bound of 127 645 on the maximal lengths of both multiplicative and completely multiplicative sequences of discrepancy 3. We also demonstrate that unrestricted discrepancy 3 sequences can be longer than 130 000
Collaborating Low Cost Micro Aerial Vehicles: A Demonstration
Full text linkIn this paper we demonstrate our Distributed Collaborative Tracking and Mapping (DCTAM) system for collaborative localization and mapping with teams of Micro-Aerial Vehicle’s MAVs. DCTAM uses a distributed architecture which allows us to run both image capture and frame-to-frame tracking on-board the MAV while offloading the more computationally demanding tasks of map creation/refinement to an offboard computer. The low computational cost of the localisation components of our system allow us to run additional software on-board such as an Extended Kalman Filter (EKF) for full state estimation and a PID-based Position Controller. This allows us to demonstrate complete cooperative autonomous operation
Mapping Analysis in Ontology-based Data Access: Algorithms and Complexity (Extended Abstract)
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Querying Over Encrypted Databases and Flexible Access Control Using Advanced Encryption Adjustment Policies
No full textSecurity, especially data privacy, is a major barrier to using outsourced servers. Recent security breaches
have shown that a stronger protection mechanism is required to keep data private from curious administrators.
Prior efforts to address this security issue have been either unfeasibly slow or operationally limited. While
most protection efforts are focused on relational databases, additional security measures are still lacking.
However, the growing number of online users and the popularity of Web 2.0 have led to storage issues and
limitations of traditional relational databases, which have led to a preference for non-relational databases.
Non-relational databases lack security features, although there have been some basic attempts to address
confidentiality.
This thesis proposes the Secure Document DataBase (SDDB) framework, which uses a proxy to protect
outsourced databases and ensure user authentication and authorisation. This thesis’s framework is based on
the CryptDB concept and Ciphertext Policy Attributes-Based Encryption. This framework allows many users
to access a single data owner’s encrypted data in an outsourced document database. This is to protect an
outsourced database’s privacy from internal attacks (i.e., an honest-but-curious administrator) and external
attacks (i.e., unauthorised users) and ensures access control at the database level. A framework secures data
querying using multi-layer encryption and adjustment policies, resulting in three major novelties. First, under
multi-layer encryption, a new secure data querying method, Ciphertext Policy Attributes Based Encryption
(CP-ABE), has been used to validate database-level data access control. Second, under adjustment policies,
Release-Aware In-Out Encryption Adjustment (RAEA) has been developed to provide a trade-off between
security and performance during query execution. RAEA is a dynamic query execution technique for
conjunctive queries that adjusts in two directions: inward and outward adjustments. Third, under adjustment
policies, sorted criteria, and an update-aware adjustments policies are developed to address information
disclosed and communication rounds during query execution.
To achieve the thesis’s objective, we built a prototype on top of MongoDB that employs all adjustment
policies in JSON format to analyse performance and security. The proposed policies were tested using
datasets produced on either local or cloud-based database servers. The produced datasets were used to assess
the scalability of the proposed policies by examining the runtime as data size increased and database server
types. Security was measured by comparing the information revealed by the proposed policies with each
other and with that of simple encryption adjustment from related work. Performance was majorly measured
through runtime for i-iii terms: (i) decryption data for inward direction; (ii) re-encryption data for outward
direction; and (iii) communication between proxy and database. Security and performance were evaluated by
comparing the proposed policies with each other and to that of simple encryption adjustment through runtime
for i-iii terms and the information revealed for security. The evaluations indicated that, when compared to
simple adjustments, the proposed policies reduced execution time, reduced communication rounds, enhanced
performance, and ensured maximum security in terms of data leakage. They are effective for scalability data
and can be used with any relational or non-relational databases
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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