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Graph homomorphisms through random walks [electronic resource]
, Article Journal of Graph Theory ; 2003, Volume 44, Issue 1, pages 15–38 ; Hajiabolhassan, Hossein ; Sharif University of Technology
Abstract
In this paper we introduce some general necessary conditions for the existence of graph homomorphisms, which hold in both directed and undirected cases. Our method is a combination of Diaconis and Saloff–Coste comparison technique for Markov chains and a generalization of Haemers interlacing theorem. As some applications, we obtain a necessary condition for the spanning subgraph problem, which also provides a generalization of a theorem of Mohar (1992) as a necessary condition for Hamiltonicity. In particular, in the case that the range is a Cayley graph or an edge-transitive graph, we obtain theorems with a corollary about the existence of homomorphisms to cycles. This, specially, provides...
Unique list-colourability and the fixing chromatic number of graphs [electronic resource]
, Article Discrete Applied Mathematics ; Volume 152, Issues 1–3, 1 November 2005, Pages 123–138 ; Hajiabolhassan, Hossein ; Sharif University of Technology
Abstract
In this paper we introduce a chromatic parameter, called the fixing chromatic number, which is related to unique colourability of graphs, in the sense that it measures how one can embed the given graph G in G∪Kt by adding edges between G and Kt to make the whole graph uniquely t-colourable. We study some basic properties of this parameter as well as its relationships to some other well-known chromatic numbers as the acyclic chromatic number. We compute the fixing chromatic number of some graph products by applying a modified version of the exponential graph construction
Circular colouring and algebraic no-homomorphism theorems
, Article European Journal of Combinatorics ; Volume 28, Issue 6, August 2007, Pages 1843–1853 ; Hajiabolhassan, Hossein ; Sharif University of Technology
Abstract
In this paper, we apply some new algebraic no-homomorphism theorems in conjunction with some new chromatic parameters to estimate the circular chromatic number of graphs. To show the applicability of the general results, as a couple of examples, we generalize a well known inequality for the fractional chromatic number of graphs and we also show that the circular chromatic number of the graph obtained from the Petersen graph by excluding one vertex is equal to 3. Also, we focus on the Johnson–Holroyd–Stahl conjecture about the circular chromatic number of Kneser graphs and we propose an approach to this conjecture. In this regard, we introduce a new related conjecture on Kneser graphs and we...
Density and power graphs in graph homomorphism problem
, Article Discrete Mathematics ; Volume 308, Issue 17, 6 September 2008, Pages 4027–4030 ; Hajiabolhassan, Hossein ; Sharif University of Technology
Abstract
We introduce two necessary conditions for the existence of graph homomorphisms based on the concepts of density and power graph. As corollaries, we obtain a lower bound for the fractional chromatic number, and we set forward elementary proofs of the facts that the circular chromatic number of the Petersen graph is equal to three and the fact that the Coxeter graph is a core
Temperature dependence study of nonocontact AFM images using molecular dynamics simulations [electronic resource]
, Article Int. Journal of Modern Physics ; 2012, Vol. 5, pp. 418-432 ; Meghdar, Ali ; Sharif University of Technology
Abstract
The effect of temperature on the noncontact atomic force microscopy (NC-AFM) surface imaging is investigated with the aid of molecular dynamics (MD) analysis based on the Sutton-Chen (SC) interatomic potential. Particular attention is devoted to the tip and sample flexibility at different temperatures. When a gold coated probe is brought close to the Au (001) surface at high temperatures, the tip and surface atoms are pulled together and their distance becomes smaller. The tip and sample atoms displacement varies in the different environment temperatures and this leads to the different interaction forces. Along this line, to study the effect of temperature on the resulting images, we have...
On the peak-to-average power of OFDM signals based on oversampling
, Article IEEE Transactions on Communications ; Volume 51, Issue 1 , 2003 , Pages 72-78 ; 00906778 (ISSN) ; Gharavi Alkhansari, M ; Hossein Khalaj, B ; Sharif University of Technology
2003
Abstract
Orthogonal frequency-division multiplexing (OFDM) introduces large amplitude variations in time, which can result in significant signal distortion in the presence of nonlinear amplifiers. In this paper, we introduce a new bound for the peak of the continuous envelope of an OFDM signal, based on the maximum of its corresponding oversampled sequence, that is shown to be very tight as the oversampling rate increases. The bound is then used to derive a closed-form probability upper bound for complementary cumulative distribution function of the peak-to-mean envelope power ratio of uncoded OFDM signals for sufficiently large numbers of subcarriers. As another application of the bound for...
On the isoperimetric spectrum of graphs and its approximations [electronic resource]
, Article Journal of Combinatorial Theory, Series B ; Volume 100, Issue 4, July 2010, Pages 390–412 ; Hajiabolhassan, Hossein ; Javadi, Ramin ; Sharif University of Technology
Abstract
In this paper1 we consider higher isoperimetric numbers of a (finite directed) graph. In this regard we focus on the nth mean isoperimetric constant of a directed graph as the minimum of the mean outgoing normalized flows from a given set of n disjoint subsets of the vertex set of the graph. We show that the second mean isoperimetric constant in this general setting, coincides with (the mean version of) the classical Cheeger constant of the graph, while for the rest of the spectrum we show that there is a fundamental difference between the nth isoperimetric constant and the number obtained by taking the minimum over all n-partitions. In this direction, we show that our definition is the...
Automatic Language Model Adaptation
, M.Sc. Thesis Sharif University of Technology ; Sameti, Hossein (Supervisor)
Abstract
Language Model plays a vital role in speech recognition systems. Restricting the search space, language models combine the result of words recognition phase with the assumptions gained from the rules and structure of words’ syntax (inflection and etc…), resulting into a more appropriate sequence of words as output. Since the language model for a specific language is not static and varies under the influence of different conditions, the problem of matching language model is being posed. In general, the aim of an adaptive language model is to offer a model capable of capturing all possible changes in the structure of words’ syntax, inflection and semantic structures. If the matching process...
Determining Success Factors of Business Accelerators in the Creation and Growth of Iranian E-Commerce Start-ups
, M.Sc. Thesis Sharif University of Technology ; Sharif, Hossein (Supervisor)
Abstract
In recent years there has been an increasing attention and significance being devoted to promotion and implementation of policies and mechanisms which foster entrepreneurship in various countries. In such a setting, many researchers and policy advisers have recommended establishment of new institutions which cause the growth in the number of entrepreneurs as well as their success rate in establishing their enterprises. The introduction of "Accelerators" and their subsequent success, have attracted the attention of many entrepreneurship stakeholders, such as entrepreneurs, investors and policy makers. In spite of the establishment and growth of accelerators in many countries, so far there has...
A new model for robust facility layout problem [electronic resource]
, Article Information Sciences, Elsevier ; Volume 278, 10 September 2014, Pages 498–509 ; Eshghi, Kourosh ; Salmani, Mohammad Hassan ; Sharif University of Technology
Abstract
The Facility Layout Problem (FLP) is the problem of locating each department in a long plant floor without any overlap between departments in order to minimize the material handling cost. The main purpose of this study is to show the effectiveness of a robust approach to solve FLP. In this study, it is assumed that the departments’ length and width are not predetermined. For modeling this kind of uncertainty, the size of each department is considered as a bounded variable and two new parameters are also introduced to implement a robust approach. Moreover, a new adaptive algorithm is designed to determine the robust layout with respect to the decision makers’ requirements. Furthermore, the...
Simulation, optimization & control of styrene bulk polymerization in a tubular reactor
, Article Iranian Journal of Chemistry and Chemical Engineering ; Volume 32, Issue 4 , 2013 , Pages 69-79 ; 10219986 (ISSN) ; Mohammad, S ; Hossein, A ; Sharif University of Technology
Jihad Danishgahi
2013
Abstract
In this paper, optimization and control of a tubular reactor for thermal bulk post-polymerization of styrene have been investigated. By using the reactor mathematical model, static and dynamic simulations are carried out. Based on an objective function including polymer conversion and polydispersity, reactor optimal temperature profile has been obtained. In the absence of model mismatch, desired product characteristic can also be obtained by applying the corresponding reactor wall or jacket temperature profile. To achieve this temperature trajectory, reactor jacket is divided into three zones and jacket inlet temperatures are used as manipulated variables. Effectiveness of the proposed...
A nine-step approach for developing and implementing an “agricultural drought risk management plan”; case study: Alamut River basin in Qazvin, Iran
, Article Natural Hazards ; Volume 102, Issue 3 , 2020 , Pages 1187-1205 ; Hosseini Hossein Abadi, F ; Sharif University of Technology
Springer
2020
Abstract
Occurrence of drought, as an inevitable natural climate feature, cannot be ceased while happening. However, costs of the consequences could be alleviated using mature scientific integrated approaches. To reduce the amount of damage, it is required to provide “Contingency” and “Mitigation” action plans. For this reason, development of efficient operating instructions for various regions based on weather conditions and field studies is needed as well as having a sophisticated understanding of socioeconomic situations. This paper describes an approach to provide the first national agricultural drought risk management plan for a river basin in Iran country as a pilot. The study lasted for...
Main and interaction effects of manufacturing variables on microstructure and fracture of solder-copper connections
, Article Engineering Failure Analysis ; Volume 139 , 2022 ; 13506307 (ISSN) ; Nourani, A ; Hossein Farrahi, G ; Sharif University of Technology
Elsevier Ltd
2022
Abstract
Finding the optimized set of manufacturing parameters to produce strong solder-copper connections requires investigating the main and interaction effects of processing variables on the joint strength and microstructure. In this study, solder joint specimens were prepared at different levels of cooling rate, time above liquidus (TAL), and soldering temperature. Mode I fracture experiments were designed and performed at a strain rate of 0.5 s−1. The fracture load remained constant from the cooling rate of 0.1 to 1.4 °C/s and then decreased by almost 34% with further increase in the cooling rate to 34 °C/s. Increasing TAL from 60 to 120 s reduced the fracture load by almost 27%, while it was...
An adaptive feedback bit allocation method for wireless systems with transmit diversity
, Article 2005 1st International Conference on Computers, Communications and Signal Processing with Special Track on Biomedical Engineering, CCSP 2005, Kuala Lumpur, 14 November 2005 through 16 November 2005 ; 2005 , Pages 181-184 ; 1424400112 (ISBN); 9781424400119 (ISBN) ; Shariatpanahi, P ; Hossein Khalaj, B ; Sharif University of Technology
2005
Abstract
Using multiple antenna transmitters and/or receivers is becoming a common method for improving spectral efficiency of wireless channels. The physical layer of the next generation of WLAN systems (802.11n) is based on a Multiple Input Multiple Output (MIMO) system which seems to be an efficient solution for defects which could occur in wireless channels. In this paper, we investigate an adaptive bit allocation method to feed the quantized channel state data back to the transmitting antennas, depending on the channel condition in a Multiple Input Single Output (MISO) scheme. We have proposed an adaptive bit allocation scheme with a significant performance improvement over the fixed bit...
Feedback bit reduction for antenna selection methods in wireless systems
, Article 2005 13th IEEE International Conference on Networks jointly held with the 2005 7th IEEE Malaysia International Conference on Communications, Kuala Lumpur, 16 November 2005 through 18 November 2005 ; Volume 1 , 2005 , Pages 229-233 ; 1424400007 (ISBN); 9781424400003 (ISBN) ; Babadi, B ; Hossein Khalaj, B ; Sharif University of Technology
2005
Abstract
A well known method to reduce the intrinsic complexity of Multiple Input Multiple Output (MIMO) systems is to choose a subset of available antennas which have stronger links than the others, in order to perform the specified MIMO algorithm. The data resulted from the antenna selection process (at the receiver side) is sent back to the transmitter side via a feedback channel. There seems to be a need to reduce the number of feedack bits, specially when the number of antennas is not small. In this paper, we investigate the problem of reducing the number of feedback bits in antenna selection techniques. We've proposed two methods using vector quantization techniques to perform feedback bit...
Calculation of the Energy Release Rate of Nano-Cracks in FCC Materials Via the Many Body Atomic Scale FEM
, M.Sc. Thesis Sharif University of Technology ; Mohammadi Shodja, Hossein (Supervisor)
Abstract
Since the classical continuum theory fails to deal with the problems associated with defects, stress concentrators, and relevant deformation phenomena in solids, alternative approaches that can detect the atomistic nature of materials' fracture are required. The deficiency of the capture the size effect which yields delusively high values for some components of the stress field right on the edge of the stress concentrators, and its weakness in describing the complex interaction between small inhomogeneities, cracks and the like when they are only a few nanometers apart, are among some of the disadvantages of the classical approach. In recent years, however, atomistic methods are emerging to...
Identifying and Rating Indicators for Measuring Iranian Entrepreneurship Ecosystem based on OECD Framework’s Domains with ANDE Method
, M.Sc. Thesis Sharif University of Technology ; Yavari, Elham (Supervisor) ; Sharif, Hossein (Co-Advisor)
Abstract
“Entrepreneurship ecosystem” is defined as the elements – individuals, organizations or institutions – apart from the individual entrepreneur that are conducive to, or inhibitive of, the choice of a person to become an entrepreneur, or the probabilities of his or her success following launch. Entrepreneurial businesses operate locally or regionally and are therefore subject to local or regional contextual influence. Moreover particularly in larger countries there can exist significant variation in industry structure and economic base across regions, emphasizing the importance of regional focus. Adopting appropriate policies at the regional level need to have a deep and accurate...
Performance analysis of the clustered caching wireless networks considering stochastic geometry approach
, Article 2016 8th International Symposium on Telecommunications, IST 2016, 27 September 2016 through 29 September 2016 ; 2017 , Pages 320-324 ; 9781509034345 (ISBN) ; Zeinalpour Yazdi, Z ; Hossein Khalaj, B ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2017
Abstract
Recently, caching of the popularity video files in the caches near to the users has been proposed as a promising way to reduce data traffic. These caches can be considered as the nodes which their transmission ranges are limited but have significant storage capacity and are known as the helper nodes. To evaluate the performance and efficiency of the caches, a suitable model for their distributions should be taken into account. Stochastic geometry is a useful tool for modeling these nodes. In this paper, the helper nodes and users are distributed according to the Poisson point process distribution. However, to manage the interferences and their access to a distributed cache, the helper nodes...
Physical-Layer schemes for wireless coded caching
, Article IEEE Transactions on Information Theory ; Volume 65, Issue 5 , 2019 , Pages 2792-2807 ; 00189448 (ISSN) ; Caire, G ; Hossein Khalaj, B ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2019
Abstract
We investigate the potentials of applying the coded caching paradigm in wireless networks. In order to do this, we investigate physical layer schemes for downlink transmission from a multiantenna transmitter to several cache-enabled users. As the baseline scheme, we consider employing coded caching on the top of max-min fair multicasting, which is shown to be far from optimal at high-SNR values. Our first proposed scheme, which is near-optimal in terms of DoF, is the natural extension of multiserver coded caching to Gaussian channels. As we demonstrate, its finite SNR performance is not satisfactory, and thus we propose a new scheme in which the linear combination of messages is implemented...
Fast estimation of propagation constants in lamellar gratings needless of solving the eigenvalue equation
, Article 27th Iranian Conference on Electrical Engineering, ICEE 2019, 30 April 2019 through 2 May 2019 ; 2019 , Pages 1342-1346 ; 9781728115085 (ISBN) ; Akbari, M ; Hossein Nekuee, S. A ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2019
Abstract
Fourier-based modal methods are among the most significant tools for accurate numerical analysis of grating structures. However, they mostly lead to time consuming and memory hungry eigenvalue problems, particularly when large dielectric constants or high contrasts are involved. We have found an asymptotic semi-empirical relationship for the propagation constants of a lamellar grating, obtained from Fourier-based modal methods. Hence, given any truncation order, it is possible to estimate propagation constants without having to solve the eigenvalue equation. We observed propagation constants only depend on permittivities, filling factors, and the unit cell size, while the dependence on the...