Wormholes in viable f(R)f(R) modified theories of gravity and Weak Energy Condition

In this work wormholes in viable $f(R)$ gravity models are analysed. We are interested in exact solutions for stress-energy tensor components depending on different shape and redshift functions. Several solutions of gravitational equations for different $f(R)$ models are examined. Found solutions imply no need for exotic material, while this need is implied in the standard general theory of relativity. Simple expression for WEC violation near the throat is derived and analysed. High curvature regime is also discussed, as well as the question of the highest possible values of Ricci scalar for which WEC is not violated near the throat, and corresponding functions are calculated for the several models. The approach here differs from the one that has been common since no additional assumptions to simplify the equations are made, and functions in $f(R)$ models are not taken to be arbitrary functions, but rather a feature of the theory that has to be evaluated on the basis of consistency with observations for the Solar System and cosmological evolution.Comment: 8 pages, 10 figures, accepted by the European Physical Journal

Creation of wormholes during the cosmological bounce

In this work we demonstrate that wormholes can in principle be naturally created during the cosmological bounce without the need for the exotic matter or any kind of additional modifications of the gravitational sector, apart from the one enabling the cosmological bounce. This result is general and does not depend on the details of the modifications of gravitational equations needed to support the bounce. To study the possible existence of wormholes around the cosmological bounce we introduce general modifications of Einstein's field equations need to support the bouncing solutions. In this regime we show that it is possible to construct a cosmological wormhole solution supported by matter, radiation and vacuum energy, satisfying the Weak Energy Condition (WEC), which asymptotically approaches the Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) metric. However, at a specific cosmological time, which depends on the parameters of the bouncing cosmological model, the WEC describing the matter needed to support such wormholes is spontaneously violated. This means that such wormholes could potentially exist in large numbers during some period around the bounce, significantly changing the causal structure of space-time, and then vanish afterwards.Comment: 17 pages, 3 figure

Determining the set of the most important components for system reliability

Mere značajnosti (Importance measures) predstavlјaju načine merenja, tj. brojčanog iskazivanja značajnosti pojedinih komponenata u sistemu sa aspekta ukupne pouzdanosti sistema. Merama značajnosti je moguće odrediti (izdvojiti) komponente najznačajnije za pouzdanost sistema. Od šezdesetih godina, kada je koncept mera značajnosti prvi put uveden, do danas postoji neprekidno interesovanje za ovu oblast, tako da se, pored primene tradicionalnih mera značajnosti, neprestano uvode i definišu nove mere radi njihove primene na specifične sisteme. Opšti nedostatak mera značajnosti, nezavisno od kategorije kojoj pripadaju, je taj što se one utvrđuju za svaku pojedinačnu komponentu, a tek nakon toga se može izdvojiti skup najznačajnijih komponenata zadate kardinalnosti...Importance measures are numerical representations of the importance of each system’s component considering total system reliability. Using importance measures, the most important components for system reliability can be determined. Since the sixties, when the concept of importance measures was first introduced, there is a constant interest in this area, so that, in addition to the traditional importance measures, new measures for specific systems observed are continually introduced and defined. The general weak point of importance measures, irrespective of the category they belong to, is that they are determined for each individual component, and only afterwards a certain number of most important components can be set aside..

INVESTIGATING THE IMPACT OF TREE-BASED NETWORK TOPOLOGY ON THE SDN CONTROLLER PERFORMANCE

Software Defined Networking (SDN) is an important technology that enables a new approach to how we develop and manage networks. SDN divides the data plane and control plane and promotes logical centralization of network control so that the controller can schedule the data in the network effectively through the OpenFlow protocol. The performance and capabilities of the controller itself are important. The impact of network topology type on controller performance can be very significant. In order to have better communication in SDN, it is essential to have an analysis of the performance of specific network topologies. In this paper, we simulate ONOS and RYU controllers and compare their different network parameters under the proposed complex custom Tree-based topology. A network topology has been designed using a Mininet emulator, and the code for topology is executed in Python. From the throughput, packet transmission rate, and latency analysis, the ONOS controller displayed better results than RYU, showing that it can respond to requests more efficiently under complex SDN topologies and traffic loads. On the contrary, the RYU controller provides better results for the less complex SDN networks

Povećava li kraća hemodijaliza rizik od smrti?

Improvement in technology has made it possible to deliver a high dialysis dose in a shorter period of time. Studies of the relationship between dialysis treatment duration and mortality have revealed that the risk of death increased significantly as the treatment time was reduced. The longer duration and increased frequency of dialysis achieve an excellent clearance of small- and middleweight toxins, enable equilibration of tissue and vascular compartments, improve appetite and permit liberalization of diet, while gentle ultrafiltration allows for better control of hypertension. Better clearance of uremic toxins, normalization of cellular and extracellular volume, and improved nutrition result in a significant decrease in morbidity and mortality of dialyzed patients. With most of dialyzers in use, adequate hemodialysis can be delivered in 4 to 5 hours, especially in a setting of maximal blood and high dialysate flow, and low access recirculation. Although controversial, the preliminary evidence available favor the use of a biocompatible membrane and more frequent or prolonged dialysis to ensure adequate removal of small- and middle molecular weight toxins, yet is preventing the loss of essential solutes.Tehnički napredak omogućio je da bolesnici dobiju veću dozu dijalize u kraćem vremenu. Kada je ispitan odnos između trajanja dijalize i smrtnosti, utvrđeno je da se rizik od smrti značajno povećao usporedno sa skraćivanjem vremena provedenog na dijalizi. Dulja dijaliza i veća učestalost dijalize pozitivno utječu na uklanjanje malih i srednje velikih toksina, omogućavaju ujednačavanje razdiobe otopina između tkiva i krvnih žila, poboljšavaju apetit i dozvoljavaju slobodniju prehranu, a polagana ultrafiltracija omogućava bolju kontrolu hipertenzije. Bolje odstranjivanje uremijskih toksina, normaliziranje staničnog i izvanstaničnog volumena te bolja prehrana smanjuju stopu pobolijevanja i smrtnosti u dijaliziranih bolesnika. Većina danas dostupnih dijalizatora omogućava, u uvjetima maksimalnog protoka krvi i visokog protoka dijalizata, a uz istodobno nizak postotak recirkulacije u krvožilnom pristupu, primjerenu hemodijalizu u trajanju od 4 do 5 sati. Iako neujednačeni, rezultati preliminarnih istraživanja upućuju na to da uporaba biokompatibilnih membrana i češća ili produžena dijaliza omogućavaju primjereno uklanjanje toksina male i srednje velike molekularne veličine, a da pritom ne dolazi do gubitka važnih supstancija