Enviament des de DRAC
http://hdl.handle.net/2117/3055
2015-11-27T08:13:59ZThe conjugacy problem for free-by-cyclic groups
http://hdl.handle.net/2117/79985
The conjugacy problem for free-by-cyclic groups
Martino, Armando; Ventura Capell, Enric
We show that the conjugacy problem is solvable in [finitely
generated free]-by-cyclic groups, by using a result of O. Maslakova
that one can algorithmically find generating sets for the fixed sub-
groups of free group automorphisms, and one of P. Brinkmann that
one can determine whether two cyclic words in a free group are
mapped to each other by some power of a given automorphism. The
algorithm effectively computes a conjugating element, if it exists. We
also solve the power conjugacy problem and give an algorithm to rec-
ognize if two given elements of a finitely generated free group are
Reidemeister equivalent with respect to a given automorphism.
2015-11-26T18:18:08ZMartino, ArmandoVentura Capell, EnricWe show that the conjugacy problem is solvable in [finitely
generated free]-by-cyclic groups, by using a result of O. Maslakova
that one can algorithmically find generating sets for the fixed sub-
groups of free group automorphisms, and one of P. Brinkmann that
one can determine whether two cyclic words in a free group are
mapped to each other by some power of a given automorphism. The
algorithm effectively computes a conjugating element, if it exists. We
also solve the power conjugacy problem and give an algorithm to rec-
ognize if two given elements of a finitely generated free group are
Reidemeister equivalent with respect to a given automorphism.The automorphism group of a free-by-cyclic group in rank 2
http://hdl.handle.net/2117/79983
The automorphism group of a free-by-cyclic group in rank 2
Bogopolski, Oleg; Martino, Armando; Ventura Capell, Enric
Let
¡
be an automorphism of a free group
F
2
of rank
2 and let
M
¡
=
F
2
o
¡
Z
be the corresponding mapping torus of
¡
.
We prove that the group
Out
(
M
¡
) is usually virtually cyclic. More-
over, we classify the cases when this group is Ønite depending on the
conjugacy class of the image of
¡
in
GL
2
(
Z
)
2015-11-26T17:54:44ZBogopolski, OlegMartino, ArmandoVentura Capell, EnricLet
¡
be an automorphism of a free group
F
2
of rank
2 and let
M
¡
=
F
2
o
¡
Z
be the corresponding mapping torus of
¡
.
We prove that the group
Out
(
M
¡
) is usually virtually cyclic. More-
over, we classify the cases when this group is Ønite depending on the
conjugacy class of the image of
¡
in
GL
2
(
Z
)Absolute-type shaft encoding using LFSR sequences with a prescribed length
http://hdl.handle.net/2117/79981
Absolute-type shaft encoding using LFSR sequences with a prescribed length
Fuertes Armengol, José Mª; Balle Pigem, Borja de; Ventura Capell, Enric
Maximal-length binary sequences have existed for a long time. They have many interesting properties, and one of them is that, when taken in blocks of n consecutive positions, they form 2n - 1 different codes in a closed circular sequence. This property can be used to measure absolute angular positions as the circle can be divided into as many parts as different codes can be retrieved. This paper describes how a closed binary sequence with an arbitrary length can be effectively designed with the minimal possible block length using linear feedback shift registers. Such sequences can be used to measure a specified exact number of angular positions using the minimal possible number of sensors that linear methods allow.
2015-11-26T17:41:12ZFuertes Armengol, José MªBalle Pigem, Borja deVentura Capell, EnricMaximal-length binary sequences have existed for a long time. They have many interesting properties, and one of them is that, when taken in blocks of n consecutive positions, they form 2n - 1 different codes in a closed circular sequence. This property can be used to measure absolute angular positions as the circle can be divided into as many parts as different codes can be retrieved. This paper describes how a closed binary sequence with an arbitrary length can be effectively designed with the minimal possible block length using linear feedback shift registers. Such sequences can be used to measure a specified exact number of angular positions using the minimal possible number of sensors that linear methods allow.Absolute type shaft encoding using LFSR sequences with prescribed length
http://hdl.handle.net/2117/79980
Absolute type shaft encoding using LFSR sequences with prescribed length
Fuertes Armengol, José Mª; Balle, Borja; Ventura Capell, Enric
Maximal-length binary sequences have been known for
a long time. They have many interesting properties, one of them
is that when taken in blocks of
n
consecutive positions they form
2
n
°
1 diÆerent codes in a closed circular sequence. This property
can be used for measuring absolute angular positions as the circle
can be divided in as many parts as diÆerent codes can be retrieved.
This paper describes how can a closed binary sequence with arbitrary
length be eÆectively designed with the minimal possible block-length,
using
linear feedback shift registers
(LFSR). Such sequences can be
used for measuring a speciØed exact number of angular positions,
using the minimal possible number of sensors that linear methods
allow
2015-11-26T17:28:19ZFuertes Armengol, José MªBalle, BorjaVentura Capell, EnricMaximal-length binary sequences have been known for
a long time. They have many interesting properties, one of them
is that when taken in blocks of
n
consecutive positions they form
2
n
°
1 diÆerent codes in a closed circular sequence. This property
can be used for measuring absolute angular positions as the circle
can be divided in as many parts as diÆerent codes can be retrieved.
This paper describes how can a closed binary sequence with arbitrary
length be eÆectively designed with the minimal possible block-length,
using
linear feedback shift registers
(LFSR). Such sequences can be
used for measuring a speciØed exact number of angular positions,
using the minimal possible number of sensors that linear methods
allowL-EncDB: a lightweight framework for privacy-preserving data queries in cloud computing
http://hdl.handle.net/2117/79976
L-EncDB: a lightweight framework for privacy-preserving data queries in cloud computing
Li, Jin; Liu, Zheli; Chen, Xiaofeng; Xhafa Xhafa, Fatos; Tan, xiao; Wong, Duncan S.
With the advent of cloud computing, individuals and organizations have become interested in moving their databases from local to remote cloud servers. However, data owners and cloud service providers are not in the same trusted domain in practice. For the protection of data privacy, sensitive data usually have to be encrypted before outsourcing, which makes effective database utilization a very challenging task. To address this challenge, in this paper, we propose L-EncDB, a novel lightweight encryption mechanism for database, which (i) keeps the database structure and (ii) supports efficient SQL-based queries. To achieve this goal, a new format-preserving encryption (FPE) scheme is constructed in this paper, which can be used to encrypt all types of character strings stored in database. Extensive analysis demonstrates that the proposed 1,-EncDB scheme is highly efficient and provably secure under existing security model.
2015-11-26T15:52:56ZLi, JinLiu, ZheliChen, XiaofengXhafa Xhafa, FatosTan, xiaoWong, Duncan S.With the advent of cloud computing, individuals and organizations have become interested in moving their databases from local to remote cloud servers. However, data owners and cloud service providers are not in the same trusted domain in practice. For the protection of data privacy, sensitive data usually have to be encrypted before outsourcing, which makes effective database utilization a very challenging task. To address this challenge, in this paper, we propose L-EncDB, a novel lightweight encryption mechanism for database, which (i) keeps the database structure and (ii) supports efficient SQL-based queries. To achieve this goal, a new format-preserving encryption (FPE) scheme is constructed in this paper, which can be used to encrypt all types of character strings stored in database. Extensive analysis demonstrates that the proposed 1,-EncDB scheme is highly efficient and provably secure under existing security model.A massive data processing approach for effective trustworthiness in online learning groups
http://hdl.handle.net/2117/79974
A massive data processing approach for effective trustworthiness in online learning groups
Miguel, Jorge; Caballé Llobet, Santiago; Xhafa Xhafa, Fatos; Prieto, Josep
This paper proposes a trustworthiness-based approach for the design of secure learning activities in online learning groups. Although computer-supported collaborative learning has been widely adopted in many educational institutions over the last decade, there exist still drawbacks that limit its potential. Among these limitations, we investigate on information security vulnerabilities in learning activities, which may be developed in online collaborative learning contexts. Although security advanced methodologies and technologies are deployed in learning management systems, many security vulnerabilities are still not satisfactorily solved. To overcome these deficiencies, we first propose the guidelines of a holistic security model in online collaborative learning through an effective trustworthiness approach. However, as learners' trustworthiness analysis involves large amount of data generated along learning activities, processing this information is computationally costly, especially if required in real time. As the main contribution of this paper, we eventually propose a parallel processing approach, which can considerably decrease the time of data processing, thus allowing for building relevant trustworthiness models to support learning activities even in real time.
2015-11-26T15:10:35ZMiguel, JorgeCaballé Llobet, SantiagoXhafa Xhafa, FatosPrieto, JosepThis paper proposes a trustworthiness-based approach for the design of secure learning activities in online learning groups. Although computer-supported collaborative learning has been widely adopted in many educational institutions over the last decade, there exist still drawbacks that limit its potential. Among these limitations, we investigate on information security vulnerabilities in learning activities, which may be developed in online collaborative learning contexts. Although security advanced methodologies and technologies are deployed in learning management systems, many security vulnerabilities are still not satisfactorily solved. To overcome these deficiencies, we first propose the guidelines of a holistic security model in online collaborative learning through an effective trustworthiness approach. However, as learners' trustworthiness analysis involves large amount of data generated along learning activities, processing this information is computationally costly, especially if required in real time. As the main contribution of this paper, we eventually propose a parallel processing approach, which can considerably decrease the time of data processing, thus allowing for building relevant trustworthiness models to support learning activities even in real time.Designing cloud-based electronic health record system with attribute-based encryption
http://hdl.handle.net/2117/79970
Designing cloud-based electronic health record system with attribute-based encryption
Xhafa Xhafa, Fatos; Li, Jingwei; Zhao, Gansen; Li, Jin; Chen, Xiaofeng; Wong, Duncan S.
With the development of cloud computing, electronic health record (EHR) system has appeared in the form of patient-centric, in which patients store their personal health records (PHRs) at a remote cloud server and selectively share them with physicians for convenient medical care. Although the newly emerged form has many advantages over traditional client-server model, it inevitably introduces patients' concerns on the privacy of their PHRs due to the fact that cloud servers are very likely to be in a different trusted domain from that of the patients. In this paper, aiming at allowing for efficient storing and sharing PHRs and also eliminating patients' worries about PHR privacy, we design a secure cloud-based EHR system, which guarantees security and privacy of medical data stored in the cloud, relying on cryptographic primitive but not the full trust over cloud servers. Based on our proposed basic EHR system, we provide several extensions including adding searchability, supporting revocation functionality and enabling efficient local decryption, which fills the gap between theoretical proposal and practical application.
2015-11-26T13:52:47ZXhafa Xhafa, FatosLi, JingweiZhao, GansenLi, JinChen, XiaofengWong, Duncan S.With the development of cloud computing, electronic health record (EHR) system has appeared in the form of patient-centric, in which patients store their personal health records (PHRs) at a remote cloud server and selectively share them with physicians for convenient medical care. Although the newly emerged form has many advantages over traditional client-server model, it inevitably introduces patients' concerns on the privacy of their PHRs due to the fact that cloud servers are very likely to be in a different trusted domain from that of the patients. In this paper, aiming at allowing for efficient storing and sharing PHRs and also eliminating patients' worries about PHR privacy, we design a secure cloud-based EHR system, which guarantees security and privacy of medical data stored in the cloud, relying on cryptographic primitive but not the full trust over cloud servers. Based on our proposed basic EHR system, we provide several extensions including adding searchability, supporting revocation functionality and enabling efficient local decryption, which fills the gap between theoretical proposal and practical application.Damage regularisation with inertia gradients
http://hdl.handle.net/2117/79968
Damage regularisation with inertia gradients
Bennett, Terry; Rodríguez Ferran, Antonio; Askes, Harm
A gradient enhanced model is formulated which simulates micro-structurally induced wave dispersion and is capable of achieving mesh objective results when modelling strain-softening materials. Length scale parameters are introduced to incorporate both micro-inertial effects and strain smoothing. The model is formulated such that all the gradient enhancement terms are contained within the kinetic energy functional, whilst all non-linear terms are linked to the strain energy functional. In addition, discretisation of the governing equations is performed in a manner such that only C0C0-continuity is required and is symmetric in the elastic range. Dispersion analysis is performed to show the correct format that a constitutive model must conform to in order that regularisation of strain-softening material models will occur. This dispersion analysis is then used to explore how the two length scales may influence the width of the zone in which strain localisation takes place. The efficacy of the model in simulating strain-softening behaviour in a mesh objective manner and the effects of wave dispersion on damage initiation and accumulation are demonstrated in numerical examples.
2015-11-26T13:34:34ZBennett, TerryRodríguez Ferran, AntonioAskes, HarmA gradient enhanced model is formulated which simulates micro-structurally induced wave dispersion and is capable of achieving mesh objective results when modelling strain-softening materials. Length scale parameters are introduced to incorporate both micro-inertial effects and strain smoothing. The model is formulated such that all the gradient enhancement terms are contained within the kinetic energy functional, whilst all non-linear terms are linked to the strain energy functional. In addition, discretisation of the governing equations is performed in a manner such that only C0C0-continuity is required and is symmetric in the elastic range. Dispersion analysis is performed to show the correct format that a constitutive model must conform to in order that regularisation of strain-softening material models will occur. This dispersion analysis is then used to explore how the two length scales may influence the width of the zone in which strain localisation takes place. The efficacy of the model in simulating strain-softening behaviour in a mesh objective manner and the effects of wave dispersion on damage initiation and accumulation are demonstrated in numerical examples.A new bipenalty formulation for ensuring time step stability in time domain computational dynamics
http://hdl.handle.net/2117/79966
A new bipenalty formulation for ensuring time step stability in time domain computational dynamics
Hetherington, Jack; Rodríguez Ferran, Antonio; Askes, Harm
2015-11-26T13:26:57ZHetherington, JackRodríguez Ferran, AntonioAskes, HarmThin-shell theory based analysis of radially pressurized multiwall carbon nanotubes
http://hdl.handle.net/2117/79965
Thin-shell theory based analysis of radially pressurized multiwall carbon nanotubes
Shima, H.; Ghosh, Susanta; Arroyo Balaguer, Marino; Iiboshi, K.; Sato, M.
The radial deformation of multiwall carbon nanotubes (MWNTs) under hydrostatic pressure is investi gated within the continuum elastic approximation. A thin shell theory, with accurate elastic constants and interwall couplings, allows us to estimate the critical pressure above which the original circular cross section transforms into radially corrugated ones. The emphasis is placed on the rigorous formula tion of the van der Waals interaction between adjacent walls, which we analyze using two different approaches. Possible consequences of the radial corrugation in the physical properties of pressurized MWNTs are also discussed.
2015-11-26T13:24:10ZShima, H.Ghosh, SusantaArroyo Balaguer, MarinoIiboshi, K.Sato, M.The radial deformation of multiwall carbon nanotubes (MWNTs) under hydrostatic pressure is investi gated within the continuum elastic approximation. A thin shell theory, with accurate elastic constants and interwall couplings, allows us to estimate the critical pressure above which the original circular cross section transforms into radially corrugated ones. The emphasis is placed on the rigorous formula tion of the van der Waals interaction between adjacent walls, which we analyze using two different approaches. Possible consequences of the radial corrugation in the physical properties of pressurized MWNTs are also discussed.