Convexity of the Energy Domain     

F.M. Reza 《Journal of The Franklin Institute》1983,315(3):179-194

A basic theorem describing the convexity of the energy domain for the general family of linear time-invariant (active or passive, reciprocal or non-reciprocal, lumped or distributed, single variable or multivariable) physical system {T} is proved.Theorem: Let F = P + jQ represent the complex energy associated with any linear physical system T (n-port). For any specified excitation of frequency s and the family of constant energy content input signals {i:∥i∥= constant}, the point F describes a convex domain in the {P;Q} plane.Part I contains a mathematical and a network theoretic proof of the foregoing theorem. In Part II the geometric nature of the energy loci for the two-ports is examined. It is shown that for all two-ports with double eigenvalves the energy domain is circular. For two-ports with distinct eigenvalves, the convex energy domain is an ellipse. The geometric characterization of this elliptic domain is examined and examples verified by computer.The concept of convexity is frequently exploited in optimization of energy in electric power system and quadratic cost functions in control theory. In this respect the central theorem of this paper, its proof and geometric ramifications should prove to be of basic interest for all linear systems.  相似文献   

Angular region: a measure of underdamped behavior     

F.M. Reza 《Journal of The Franklin Institute》1982,314(3):191-202

The natural modes of an underdamped dynamical system are given by the characteristic numbers of the quadratic operator pencil where the operator A depends on the dissipative and reactive elements of the system, while B depends solely on the reactive elements. The operator P(s) for every applied stimulus vector signal x must satisfy: A measure of underdamped behaviour is suggested by predetermining an angular region |φ| containing all natural modes of the system, When a comparison between positive operators A and B is available, say B²=KA, then The paper is motivated by Duffin-Krein-Gohberg's earlier mathematical contributions.  相似文献   

Defending recommender systems by influence analysis     

Mohammad Amin Morid  Mehdi Shajari  Ali Reza Hashemi 《Information Retrieval》2014,17(2):137-152

Collaborative filtering (CF) is a popular method for personalizing product recommendations for e-commerce applications. In order to recommend a product to a user and predict that user’s preference, CF utilizes product evaluation ratings of like-minded users. The process of finding like-minded users forms a social network among all users and each link between two users represents an implicit connection between them. Users having more connections with others are the most influential users. Attacking recommender systems is a new issue for these systems. Here, an attacker tries to manipulate a recommender system in order to change the recommendation output according to her wish. If an attacker succeeds, her profile is used over and over again by the recommender system, making her an influential user. In this study, we applied the established attack detection methods to the influential users, instead of the whole user set, to improve their attack detection performance. Experiments were conducted using the same settings previously used to test the established methods. The results showed that the proposed influence-based method had better detection performance and improved the stability of a recommender system for most attack scenarios. It performed considerably better than established detection methods for attacks that inserted low numbers of attack profiles (20–25 %).  相似文献   

Characterization and Classification of Mesenchymal Stem Cells in Several Species Using Surface Markers for Cell Therapy Purposes     

Hori Ghaneialvar  Leila Soltani  Hamid Reza Rahmani  Abbas Sahebghadam Lotfi  Masoud Soleimani 《Indian journal of clinical biochemistry : IJCB》2018,33(1):46-52

Mesenchymal stem cells are multipotent cells capable of replicating as undifferentiated cells, and have the potential of differentiating into mesenchymal tissue lineages such as osteocytes, adipocytes and chondrocytes. Such lineages can then be used in cell therapy. The aim of present study was to characterize bone marrow derived mesenchymal stem cells in four different species, including: sheep, goat, human and mouse. Human bone-marrow mesenchymal stem cells were purchased, those of sheep and goat were isolated from fetal bone marrow, and those of mouse were collected by washing bone cavity of femur and tibia with DMEM/F12. Using flow-cytometry, they were characterized by CD surface antigens. Furthermore, cells of third passage were examined for their osteogenic and adipogenic differentiation potential by oil red and alizarin red staining respectively. According to the results, CD markers studied in the four groups of mesenchymal stem cells showed a different expression. Goat and sheep expressed CD44 and CD166, and weakly expressed CD34, CD45, CD105 and CD90. Similarly, human and mouse mesenchymal cells expressed CD44, CD166, CD105 and CD90 whereas the expression of CD34 and CD45 was negative. In conclusion, although all mesenchymal stem cells display plastic adherence and tri-lineage differentiation, not all express the same panel of surface antigens described for human mesenchymal stem cells. Additional panel of CD markers are necessary to characterize regenerative potential and possible application of these stem cells in regenerative medicine and implantology.  相似文献