This dissertation considers a subclass of sum-of-ratios fractional combinatorial optimization problems (FCOPs) whose linear versions admit polynomial-time exact algorithms. This topic lies in the intersection of two scarcely researched areas of fractional programming (FP): sum-of-ratios FP and combinatorial FP. Although not extensively researched, the sum-of-ratios problems have a number of important practical applications in manufacturing, administration, transportation, data mining, etc. Since even in such a restricted research domain the problems are numerous, the main focus of this dissertation is a mathematical programming study of the three, probably, most classical FCOPs: Minimum Multiple Ratio Spanning Tree (MMRST), Minimum Multiple Ratio Path (MMRP) and Minimum Multiple Ratio Cycle (MMRC). The first two problems are studied in detail, while for the other one only the theoretical complexity issues are addressed. The dissertation emphasizes developing solution methodologies for the considered family of fractional programs. The main contributions include: (i) worst-case complexity results for the MMRP and MMRC problems; (ii) mixed 0-1 formulations for the MMRST and MMRC problems; (iii) a global optimization approach for the MMRST problem that extends an existing method for the special case of the sum of two ratios; (iv) new polynomially computable bounds on the optimal objective value of the considered class of FCOPs, as well as the feasible region reduction techniques based on these bounds; (v) an efficient heuristic approach; and, (vi) a generic global optimization approach for the considered class of FCOPs. Finally, extensive computational experiments are carried out to benchmark performance of the suggested solution techniques. The results confirm that the suggested global optimization algorithms generally outperform the conventional mixed 0{1 programming technique on larger problem instances. The developed heuristic approach shows the best run time, and delivers near-optimal solutions in most cases.

Microalgae produce large quantities of lipids that can be used for biofuel feedstock. The goal of this project was to determine the effect of several engineering and management parameters on the productivity of microalgae cultivated in large, outdoor facilities. The specific objectives were focused on the effects of inoculation ratios; the effects of light, temperature, and culture depth on growth; and the minimum circulation velocity necessary to maintain growth and minimize settling in open ponds. Microalgae must first be cultured in smaller quantities before the raceway is inoculated for optimized growth. Concentration ratios are defined as the ratio of the volume of microalgae inoculum to the volume of new growth media. The microalgae species used was Nannochloris oculata (UTEX #LB 1998). Inoculation ratios studied varied from 1:1 to 1:32 and were grown in 500 mL Erlenmeyer flasks. The study found that 1:16 and 1:32 were too dilute, while the 1:8 concentration had the largest growth rate. Determination of the effects of temperature, light intensity, and cultivation depth is critical to maintaining healthy cultures. Excess light intensity can result in photoinhibition and temperatures above the maximum growing tolerance can have detrimental effects. These factors can affect growth and evidence suggests an interaction that exacerbates these effects. In an outdoor culture there are few practical control variables other than pond depth. As cultivation depth increases, the algae undergo "selfshading" and the increased cultivation volume hinders temperature changes. Scaled raceway ponds were maintained at 10.16 cm (4 inch) and 13.97 cm (5.5 inch) depths. The species used was Nannochloris oculata and it was found to grow best at 785 micromol m⁻² s⁻¹m^-2 s^-1, 20°C, and 10.16 cm. Diatoms are attractive because of high growth rates, faster lipid production, and greater cell density. The latter promotes rapid settling once mixing has stopped. Because of the silica cell wall composition, diatoms are believed to be more susceptible to shear forces which can result in lysis. Determining the natural settling rate to the minimum channel velocity relationship in cultivation ponds was the objective. No flocculants/coagulants were added which created a case of "natural" settling. Four pennate Nitzschia sp. and one centric diatom were tested in a jar tester. There was no significant difference in settling times between the species. The mean settling time was 4.55 cm min⁻¹ and the minimum channel velocity was determined to be 10.12 cm min⁻¹.

In order to reduce default risk, financial institutions have been investigating into credit ratings of companies, which they want to give credit to. This research tries to give a method for financial institutions to differentiate between default and normal company with financial ratios, which is already announced in their seasonal financial reports. The samples are abstracted from security markets, and restricted to building companies. With Discriminant analysis and Logistic regression models, financial institutions can estimate what company may become into default situation and others stay in good condition. According to this research, financial ratios that can be used to discriminate between default and normal companies are: net worth ratio and short-turn borrowing/liquid asset and asset turnover and gross profit margin. It can also be described with asset turnover and gross profit margin if default risk is been estimated.

In recent years, people pay much attention to derivative pricing subject to credit risk. In this paper, we proposed an autoregressive time series model of log odds ratios to price derivatives. Examples of the proposed model are given via the structural and reduced form approaches. Pricing formulae of the proposed time series models are derived for bonds and options. Furthermore, simulation studies are performed to confirm the accuracy of derived formulae.

Title from PDF of title page. / Document formatted into pages; contains 93 pages / Thesis (M.S.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Coral-based climate studies commonly use elemental ratios and stable isotopes of coral skeletons to address seawater temperature and hydrologic balance issues in the tropical surface oceans. Replication, or cross-checking, a standard technique used to assess the fidelity of proxy records in paleoclimatology has not been widely applied in coral-based climate studies, primarily because of the time and cost associated with generating multiple records from a single reef site. Modern and paleoclimate reconstructions based on a single proxy-coral record from a site may contain errors if individual corals from the same reef record different geochemical signals. In this study we perform a replication test using elemental ratios and stable isotopes in three Porites corals from New Caledonia. / ABSTRACT: The reef complex offshore Amédée Island, New Caledonia is an ideal site to perform a coral replication test because instrumental sea surface temperatures (SST) and sea surface salinity measurements (SSS) have been made there for over 25 years. In this study, we compare sub-monthly resolved, geochemical variations (Sr/Ca, d18O and d13C) in three Porites lutea coral heads, located 500 m apart, with the instrumental SST and SSS records over the interval 1992-1967. The monthly coral Sr/Ca and d18O time series are well correlated to each other (r=0.86, p[.0001) and to the monthly instrumental SST record (r= -0.86, p[.0001, coral Sr/Ca to SST; r= -0.77, p[.0001, coral d18O to SST). The three, sub-monthly resolved, 30-year coral Sr/Ca-SST time series have mean SST values that agree within 0.2oC with the instrumental mean SST value. A similar comparison for the coral d18O-SST records indicate a maximum difference between predicted and observed mean SST of 0.5°C. / ABSTRACT: Analysis of the monthly climatological means also indicates that Sr/Ca-SST records closely match the instrumental SST record ±0.4°C; a similar comparison using the d18O-SST record yields an average offset of ±0.6°C between observed and predicted monthly SST. Stacking the three records to form composite Sr/Ca-SST and d18O-SST records does not appreciably improve the goodness of fit between the proxy and instrumental SST records; hence a coral-based proxy climate record from a single coral accurately reflects the observed record of climate variability at this locality. These results support the concept that high fidelity climate records can be generated using a single coral core. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Core samples were collected in September 1995 from live coral heads of Montastraea annularis at Bird Key reef in the Dry Tortugas, Florida (24 degrees 55 minutes N, 82 degrees 92 minutes W). Four 4 mm-thick coral slabs from two cores were continuously sampled at 12 samples per year (0.025 cm per sample for Core 31, 0.023 cm per sample for Core 35). Visual inspection of X-radiographs indicates an average skeletal extension rate of about 3 mm per year in Bird Key corals. The goal of this study was to perform a replication test in Montastraea annularis by using elemental and stable isotopes from four coral slabs from two different coral heads to address the following questions: 1) how well do geochemical signals replicate within a single coral head, 2) how well do geochemical signals replicate from two different cores from the same coral head, 3) how well do geochemical signals replicate from two coral heads from the same general area, and 4) do growth effects influence the geochemistry of slow-growing corals at the Dry Tortugas? Geochemical variations versus depth and time of all coral records show strong seasonal cyclicity. Variations in d18O in the suite of Bird Key coral records replicate the best; d13C and Sr/Ca variations replicate less well. For example, differences in the mean Sr/Ca record from two different coral heads are large (0.179 mmol/mol for BK31B-BK35CC; 0.196 mmol/mol for BK31C-BK35CC; ~4 degrees C) and nearly 4 times greater than analytical precision. Therefore, caution must be exercised in interpreting Sr/Ca-SST records in Montastraea annularis. Mean differences in coral d18O for all records, on the other hand, are within analytical precision and translate to temperature differences of less than 0.5 degrees C. Robust d18O values among cores that co-vary with a significant level of agreement further point to this proxy being more reliable than Sr/Ca. Because of its skeletal complexity, drilling difficulty, and large bio-geological error for Sr/Ca, Montastraea annularis seems poorly suited for coral-based Sr/Ca-SST studies. However, the species must be studied to understand tropical Atlantic interannual-decadal scale variability, so further assessment is warranted.

In the first phase of this project, four decades of monthly resolved geochemical variations from two massive heads of Montastraea were used to explore the reproducibility of the geochemical signal in these two corals from Looe Key, Florida. The coral d18O and d13C records of the two corals have statistically indistinguishable mean values, which is not the case for the coral Sr/Ca records implying that nonenvironmental factors are influencing coral Sr/Ca. Calibration equations relating coral geochemistry variations to environmental variations at Looe Key are different from previously published equations for Montastraea. These calibration differences are not related to growth-related kinetic effects, but may reflect variations in seawater chemistry in the coastal waters of the Florida Keys. Additional studies are needed to identify the causes of the observed geochemical variability. In the second phase of this study, fourteen decades of monthly resolved geochemical variations in another Montastraea coral from Looe Key, Florida were compared to records of sea-surface temperature (SST). Coral Sr/Ca and d18O variations have a weak relationship with variations in SST and skeletal extension rates; however, many events in the Sr/Ca and d18O records are coincident with anomalies in SST, growth, or precipitation. Strong coupling exists between Sr/Ca and d18O in both anomaly and mean annual perspectives, which reflects the combined influence of SST and growth related processes on the geochemical signal. Separating these impacts proved to be problematic due to modest agreements with each forcing variable. In the final phase of this study, geochemical records from three, mid-Holocene(~5 ka) fossil Montastraea corals from the Dry Tortugas, Florida were compared with geochemical records from modern Montastraea corals from the same region to investigate temporal changes in climate. Stable isotopic records show significant changes through time, which can be interpreted in terms of environmental variation; however, large inter-coral variability between modern specimens of Montastraea precludes meaningful assessment of Sr/Ca. The pattern and mean d18O values in the fossil corals reflects changes in both temperature and salinity are reminiscent of centennial-scale variability present in other records from this region.

Η παρούσα εργασία έχει ως σκοπό την ανάλυση μέσω αριθμοδεικτών που σχετίζονται με τις οικονομικές καταστάσεις των ξενοδοχειακών μονάδων της Ελλάδας. Η περίοδος μελέτης είναι το διάστημα 2009-2011, την περίοδο δηλαδή όπου η Ελλάδα διέρχονταν σε ύφεση και οικονομική κρίση. Για την ανάλυση λήφθηκαν υπόψη 60 ξενοδοχεία από διάφορες περιοχές της χώρας ώστε να καλυφθεί όσο το δυνατόν μεγαλύτερο γεωγραφικό μήκος. Σκοπός της εργασίας είναι να επιβεβαιώσει τις μελέτες του ΣΕΤΕ και του Παγκόσμιου Οργανισμού Τουρισμού, ότι ο ελληνικός τουρισμός κατά την διάρκεια της οικονομικής κρίσης υπέστη καθίζηση και οι εταιρίες του κλάδου λειτουργούσαν με ζημίες ή αναγκάζονταν σε παύση των λειτουργιών τους. / In the present essay an attempt is made in order to illustrate the impact of current economic crisis on the financial statements in hospitality sector in Greece. In order to accomplish this aim, 6 financial ratios of 60 hotels, for the period 2009-2011, were taken into account. The ratios measured are the liquidity, the solvency ratio, the ROA, the ROE, the profit margin and the debt ratio. The results showed that the business activity of Greek hotels have affected by the crisis, since in their majority have been recorded losses of the transactions or zero profits. The paradox of this study is that despite the economic crisis, the solvency ratio of Greek hotels is high enough.

Τα διαθέσιμα δεδομένα που αφορούν την κατακόρυφη διάταξη επιταχυνσιογράφων της Πάτρας (VA-1) περιλαμβάνουν περισσότερες από 1000 σεισμικές καταγραφές καθώς και στοιχεία για την, σχετικά σύνθετη, στρωματογραφία και την κατανομή VS–βάθος στη θέση της εγκατάστασης. Στην παρούσα Διατριβή υπολογίζονται οι (εμπειρικές) συναρτήσεις μεταφοράς της κίνησης στις στάθμες των υπογείων οργάνων και εκτιμώνται τα χαρακτηριστικά εδαφικής απόκρισης (δεσπόζουσες περίοδοι και ενίσχυση). Διαπιστώνεται ότι οι φασματικοί λόγοι H/V των επιφανειακών καταγραφών παρέχουν τη δυνατότητα της αποτελεσματικής αναγνώρισης των καταγραμμένων χαρακτηριστικών της κίνησης. / The available data from the Patras accelerograph vertical array (VA-1) include more than a thousand of seismic recordings as well as information on the relatively complex soil stratigraphy and distribution of VS vs depth at the location of installation. In this Thesis the (empirical) transfer functions of motion at the levels of underground instruments are estimated and the characteristics of soil response (dominant periods and amplification) are assessed. It is found that the calculated spectral ratios H/V of seismic recordings at the ground surface can provide an efficient tool for identification of the recorded motion characteristics.

The problem of finding a schedule with the lowest makespan in the class of all flowtime-optimal schedules for parallel identical machines is an NP-hard problem. Several approximation algorithms have been suggested for this problem. We focus on algorithms that are fast and easy to implement, rather than on more involved algorithms that might provide tighter approximation bounds. A set of approaches for proving conjectured bounds on performance ratios for such algorithms is outlined. These approaches are used to examine Coffman and Sethi's conjecture for a worst-case bound on the ratio of the makespan of the schedule generated by the LD algorithm to the makespan of the optimal schedule. A significant reduction is achieved in the size of a hypothesised minimal counterexample to this conjecture.