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Modification, development, application and computational experiments of some selected network, distribution and resource allocation models in operations researchNyamugure, Philimon January 2017 (has links)
Thesis (Ph.D. (Statistics)) -- University of Limpopo, 2017 / Operations Research (OR) is a scientific method for developing quantitatively
well-grounded recommendations for decision making. While it is true that it
uses a variety of mathematical techniques, OR has a much broader scope. It is
in fact a systematic approach to solving problems, which uses one or more analytical
tools in the process of analysis. Over the years, OR has evolved through
different stages. This study is motivated by new real-world challenges needed
for efficiency and innovation in line with the aims and objectives of OR – the
science of better, as classified by the OR Society of the United Kingdom. New
real-world challenges are encountered on a daily basis from problems arising
in the fields of water, energy, agriculture, mining, tourism, IT development,
natural phenomena, transport, climate change, economic and other societal requirements.
To counter all these challenges, new techniques ought to be developed.
The growth of global markets and the resulting increase in competition
have highlighted the need for OR techniques to be improved. These developments,
among other reasons, are an indication that new techniques are needed
to improve the day-to-day running of organisations, regardless of size, type and
location.
The principal aim of this study is to modify and develop new OR techniques
that can be used to solve emerging problems encountered in the areas of linear
programming, integer programming, mixed integer programming, network
routing and travelling salesman problems. Distribution models, resource allocation
models, travelling salesman problem, general linear mixed integer
ii
programming and other network problems that occur in real life, have been
modelled mathematically in this thesis. Most of these models belong to the
NP-hard (non-deterministic polynomial) class of difficult problems. In other
words, these types of problems cannot be solved in polynomial time (P). No general
purpose algorithm for these problems is known. The thesis is divided into
two major areas namely: (1) network models and (2) resource allocation and
distribution models. Under network models, five new techniques have been developed:
the minimum weight algorithm for a non-directed network, maximum
reliability route in both non-directed and directed acyclic network, minimum
spanning tree with index less than two, routing through 0k0 specified nodes,
and a new heuristic to the travelling salesman problem. Under the resource
allocation and distribution models section, four new models have been developed,
and these are: a unified approach to solve transportation and assignment
problems, a transportation branch and bound algorithm for the generalised assignment
problem, a new hybrid search method over the extreme points for
solving a large-scale LP model with non-negative coefficients, and a heuristic
for a mixed integer program using the characteristic equation approach. In
most of the nine approaches developed in the thesis, efforts were done to compare
the effectiveness of the new approaches to existing techniques. Improvements
in the new techniques in solving problems were noted. However, it was
difficult to compare some of the new techniques to the existing ones because
computational packages of the new techniques need to be developed first. This
aspect will be subject matter of future research on developing these techniques
further. It was concluded with strong evidence, that development of new OR
techniques is a must if we are to encounter the emerging problems faced by the
world today.
Key words: NP-hard problem, Network models, Reliability, Heuristic, Largescale
LP, Characteristic equation, Algorithm.
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In-silico Modeling of Lipid-Water Complexes and Lipid BilayersJadidi, Tayebeh 21 October 2013 (has links)
In the first part of the thesis, the molecular structure and electronic properties of phospholipids at the single molecule level and also for a monolayer structure are investigated via ab initio calculations under different degrees of hydration. The focus of the study is on phosphatidylcholines, in particular dipalmitoylphosphatidylcholine (DPPC), which are the most abundant phospholipids in biological membranes. Upon hydration, the phospholipid shape into a sickle-like structure. The hydration dramatically alters the surface potential, dipole and quadrupole moments of the lipids, and probably guides the interactions of the lipids with other molecules and the communication between cells. The vibrational spectrum of DPPC and DPPC-water complexes are completely assigned and it is shown that water hydrating the lipid head groups enables efficient energy transfer across membrane leaflets on sub-picosecond time scales. Moreover, the vibrational modes and lifetimes of pure and hydrated DPPC lipids, at human body temperature, are estimated by performing ab initio molecular dynamics simulations. The vibrational modes of the water molecules close to the head group of DPPC are active in the frequency range between 0.5 - 55 THz, with a peak at 2.80 THz in the energy spectrum. The computed lifetimes for the high-frequency modes agree well with recent data measured at room temperature, where high-order phonon scattering is not negligible. The structure and auto-ionization of water at the water-phospholipid interface are investigated by ab initio molecular dynamics and ab initio Monte Carlo simulations using local density approximation and generalized gradient approximation for the exchange-correlation energy functional. Depending on the lipid head group, strongly enhanced ionization is observed, leading to dissociation of several water molecules into H+ and OH- per lipid. The results can shed light on the phenomena of the high proton conductivity along membranes that has been reported experimentally. In the second part of the thesis, Monte Carlo simulations of the lipid bilayer, on the basis of a coarse grained model, are performed to gain insight into the mechanical properties of planar lipid bilayers. By using a rescaling method, the Poisson's ratio is calculated for different phases. Additional information on the bending rigidity, determined from height fluctuations on the basis of the Helfrich Hamiltonian, allows for calculation of the Young's modulus for each phase. In addition, the free energy barrier for lipid flip-flop process in the fluid and gel phases are estimated. The main rate-limiting step to complete a flip-flop process is related to a free energy barrier that has to be crossed in order to reach the center of the bilayer. The free energy cost for performing a lipid flip-flop in the gel phase is found to be five times greater than in the fluid phase, demonstrating the rarity of such events in the gel phase. Moreover, an energy barrier is estimated for formation of transient water pores that often precedes lipid translocation events and accounts for the rate-limiting step of these pore-associated lipid translocation processes.
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