A. James Clark School of Engineering
http://hdl.handle.net/1903/1654
2017-05-27T04:19:56ZOn The Number of Unlabeled Bipartite Graphs
http://hdl.handle.net/1903/19186
On The Number of Unlabeled Bipartite Graphs
Atmaca, Abdullah; Oruc, Yavuz A
Let $I$ and $O$ denote two sets of vertices, where $I\cap O =\Phi$, $|I| = n$, $|O| = r$, and $B_u(n,r)$ denote the set of unlabeled graphs whose edges connect vertices in $I$ and $O$. It is shown that the following two-sided equality holds.
$\displaystyle \frac{\binom{r+2^{n}-1}{r}}{n!} \le |B_u(n,r)| \le 2\frac{\binom{r+2^{n}-1}{r}}{n!} $
This paper describes a result that has been obtained in joint work with Abdullah Atmaca of Bilkent University, Ankara, Turkey
2016-01-01T00:00:00ZAn Expectation Maximization Approach to Revenue Management on Rail Ticket Data
http://hdl.handle.net/1903/19104
An Expectation Maximization Approach to Revenue Management on Rail Ticket Data
Kaushik, Kartik
In the world of sale of perishable commodities without regulation, competition causes cut-throat pricing and heavy discounts for the commodity. Even though this is beneficial to the customers, the companies that offer the commodity have to be careful to prevent the offered discounts and cut-throat pricing from cutting into their profits. The science of managing revenues in such scenarios is loosely termed as Revenue Management (RM). RM holds its roots to the competition generated in the American airline industry after deregulation. Since then, it has spread to virtually all industries that deal with perishable commodities such as hotel and hospitality, rental vehicles, and all forms of long distance public transportation, even freight.
The commodities in these industries refer to the items for sale. In a hotel, it may be rooms of different classes and sizes; in vehicle rentals, cars; and in all forms of long distance transportation, seating space. Perishability of these commodities can be understood simply by the fact that after a certain date, a certain commodity will not be available. In long distance transportation, it is easy to imagine that the seats on a vehicle (plane, bus, train or ferry) will not be available after the vehicle has departed on its way. Similarly rooms in a hotel or cars with a rental agency will loose value the longer they are kept empty or unused. The goal of modern day RM is, therefore, to ensure profitable sales of such commodities, such that they are priced at better rates than the competition.
This thesis attempts to apply the theory of Expectation Maximization (EM) to the purchase data from railway industry in a attempt to better the existing pricing logic. The EM algorithm used here was developed by Dr. Kalyan Talluri and Dr. Gareth van Ryzin in their seminal paper published in 2004. In that paper the authors develop the algorithm, derive the mathematics that powers it and apply it to test data sets to prove that it out performs the current industry standard. However, application of that method to a real dataset has never been done, which is the goal of this thesis.
We find, and document herewith, the issues that resulted from applying the EM algorithm directly to the data. Mainly, assumptions in the EM algorithm required heavy data clean up, after which it was found that the results were neither satisfactory nor useful. The reasons for the failure of the model are examined in detail, the primary reason being lack of identifiability in the data. To conclude, the EM algorithm needs substantial modification or additional data in order to lose certain debilitating assumptions and make it more general or reduce the identifiability problem of the data.
2016-01-01T00:00:00ZEffects of Tidal Forces on the Minimum Energy Configurations of the Full Three Body Problem
http://hdl.handle.net/1903/19103
Effects of Tidal Forces on the Minimum Energy Configurations of the Full Three Body Problem
Levine, Edward
We investigate the evolution of minimum energy configurations for the Full Three Body Problem (3BP). A stable ternary asteroid system will gradually become unstable due to the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect and an unpredictable trajectory will ensue. Through the interaction of tidal torques, energy in the system will dissipate in the form of heat until a stable minimum energy configuration is reached. We present a simulation that describes the dynamical evolution of three bodies under the mutual effects of gravity and tidal torques. Simulations show that bodies do not get stuck in local minima and transition to the predicted minimum energy configuration.
2016-01-01T00:00:00ZDURABILITY AND OPTIMIZATION OF SOFC COMPOSITE CATHODES
http://hdl.handle.net/1903/19102
DURABILITY AND OPTIMIZATION OF SOFC COMPOSITE CATHODES
Painter, Albert Steven
The combination of the conventional cathode material, La0.8Sr0.2MnO3-𝛿 (LSM), and exceptional oxygen ion conducting material, (Er0.2Bi0.8)2O3 (ESB), has shown promise as a potential candidate for low temperature solid oxide fuel cell (LT-SOFC) cathodes. Though the initial performance of this composite is encouraging, the long-term stability of LSM-ESB has yet to be investigated. Here electrochemical impedance spectroscopy (EIS) was used to in situ monitor the durability of LSM-ESB at typical LT-SOFC operation temperatures. The degradation rate as a function of aging time was extracted based on the EIS data. Post analysis suggests that below 600 °C the order-disorder transition of ESB limits the performance due to a decrease in the oxygen incorporation rate. Above 600°C, the formation of secondary phases, identified as Mn-Bi-O, is the major performance degradation mechanism. Furthermore, the relative particle size of the LSM to ESB was optimized to minimize long-term degradation in cathode performance.
2016-01-01T00:00:00Z