Darbe nagrinėjamas dvimatis atstatymo procesas. Gaunami jo integralinių lygčių ir laplaso transformacijų pavidalai, bei jų asimptotikos. / In graduate research a two-dimentional renewal process. The integral differential equation of renewal function, its Laplase transform and asymptotic's was found.

Šiame darbe atliekami stochastinių minimumų asimptotiniai tyrimai. Įrodomos minimumų ribinės teoremos tuo atveju, kai tiesinis normalizavimas neduoda neišsigimusių ribinių skirstinių, tokiu atveju taikau netiesinį minimumų normalizavimą. Konkretaus skirstinio atveju randamos netiesinės normalizavimo funkcijos, kurių pagalba yra gaunami minimumų klasikiniai ribiniai skirstiniai. Įrodoma Perkėlimo teorema netiesiniam normalizavimui. Darbo tikslai: • ištirti netiesinio normalizavimo reikalingumą; • išanalizuoti netiesinio normalizavimo galimybes minimumų schemoje. Darbo uždaviniai: • parinkti netiesinio normalizavimo funkciją konkretaus skirstinio atveju; • gauti ribinius klasikinius skirstinius, kai minimumai normalizuojami netiesiškai; • įvertinti konvergavimo greitį ribinėse teoremose; • atlikti aproksimavimo paklaidų kompiuterinę analizę. / This paper is the asymptotic analysis of stochastic minima. Proofs of minima limit theorems are provided for cases, when linear normalization does not give non-degenerate limit distributions. In this cases, non-linear minima normalization is used. For a specific distribution, non-linear normalization functions are calculated, which are then used to get classic limit distributions for minima. Objectives: • Examine the necessity of non-linear normalization; • Analyze the possibilities for non-linear normalization in minimum pattern. Tasks: • Choose non-linear normalization function for a specific distribution; • Get classic limit distributions, where minima are normalized non-linearly; • Investigate the rate of convergence within the limit theorems; • Perform computer-based analysis of approximation errors.

The increasing concentration of CO_(2) has been linked to global warming and changes in climate. Geologic sequestration of CO_(2) in deep saline aquifers is a proposed greenhouse gas mitigation technology with potential to significantly reduce atmospheric emissions of CO_(2). Feasibility assessments of proposed sequestration sites require realistic and computationally efficient models to simulate the subsurface pressure response and monitor the injection process, and quantify the risks of leakage if there is any. This study investigates the possibility of obtaining closed form expressions for spatial distribution of CO_(2) injected in brine aquifers and gas reservoirs. Four new semi-analytical solutions for CO_(2) injection in brine aquifers and gas reservoirs are derived in this dissertation. Both infinite and closed domains are considered in the study. The first solution is an analysis of CO_(2) injection into an initially brine-filled infinite aquifer, exploiting self–similarity and matched asymptotic expansion. The second is an expanding to the first solution to account for CO_(2) injection into closed domains. The third and fourth solutions are analyzing the CO_(2) injection in infinite and closed gas reservoirs. The third and fourth solutions are derived using Laplace transform. The brine aquifer solutions accounted for both Darcyian and non-Darcyian flow, while, the gas reservoir solutions considered the gas compressibility variations with pressure changes. Existing analytical solutions assume injection under constant rate at the wellbore. This assumption is problematic because injection under constant rate is hard to maintain, especially for gases. The modeled injection processes in all aforementioned solutions are carried out under constant pressure injection at the wellbore (i.e. Dirichlet boundary condition). One major difficulty in developing an analytical or semi-analytical solution involving injection of CO_(2) under constant pressure is that the flux of CO_(2) at the wellbore is not known. The way to get around this obstacle is to solve for the pressure wave first as a function of flux, and then solve for the flux numerically, which is subsequently plugged back into the pressure formula to get a closed form solution of the pressure. While there is no simple equation for wellbore flux, our numerical solutions show that the evolution of flux is very close to a logarithmic decay with time. This is true for a large range of the reservoir and CO_(2) properties. The solution is not a formation specific, and thus is more general in nature than formation-specific empirical relationships. Additionally, the solution then can be used as the basis for designing and interpreting pressure tests to monitor the progress of CO_(2) injection process. Finally, the infinite domain solution is suitable to aquifers/reservoirs with large spatial extent and low permeability, while the closed domain solution is applicable to small aquifers/reservoirs with high permeability.

The thesis is based on the use of mathematical modeling and analysis to gain insightinto the transmission dynamics of malaria in a community. A new deterministic model for assessing the role of age-structure on the disease dynamics is designed. The model undergoes backward bifurcation, a dynamic phenomenon characterized by the co-existence of a stable disease-free and an endemic equilibrium of the model when the associated reproduction number is less than unity. It is shown that adding age-structure to the basic model for malaria transmission does not alter its essential qualitative dynamics. The study is extended to incorporate the use of anti-malaria drugs. Numerical simulations of the extended model suggest that for the case when treatment does not cause drug resistance (and the reproduction number of each of the two strains exceed unity), the model undergoes competitive exclusion. The impact of various effectiveness levels of the treatment strategy is assessed.

In this thesis, a method of distortion analysis of electromagnetic field sensors using orthogonal polynomial subspaces is presented. The effective height of the sensor is viewed as the impulse response of a linear system. The impulse response corresponds to a linear transformation which maps every electromagnetic incident field waveform to a received voltage waveform. Hermite and Laguerre orthogonal polynomials are used as the basis sets for the subspace of incident electromagnetic field waveforms. Using the selected basis set, a transformation matrix is calculated for the sensors. The transformation matrices are compared to a reference transformation matrix as a measure of distortion. The transformation matrices can describe the sensor behavior up to a certain frequency range. The limits on this frequency range are investigated for both Hermite-Gauss and Laguerre functions. The unique property of Laguerre functions is used to prove that the transformation matrix has a particular pattern. This method is applied on case studied sensors both in computer simulation and measurements.

With new radioactive isotope accelerators coming online in the next decade, the problem of extracting reliable nuclear structure information from reactions with unstable nuclei deserves considerable attention. A method has been proposed to determine spectroscopic factors (SFs) using the asymptotic normalization coefficient (ANC) to fix the external contribution of a nonperipheral reaction, reducing the uncertainty in the SF. The ¹⁵C[left right arrow]¹⁴C+n system was chosen as a test case for this new method. The direct neutron capture rate on ¹⁴C is important for a variety of topics of interest in astrophysics, and the ANC for ¹⁵C[left right arrow]¹⁴C+n was also used to calculate this reaction rate. The objective of the first part of this work was to find the ANC for ¹⁵C[left right arrow]¹⁴C+n. This was done in two independent experiments. First, the heavy ion neutron transfer reaction ¹³C(¹⁴C,¹⁵C)¹²C was measured at 12 MeV/nucleon. Second, the inverse kinematics reaction d(¹⁴C,p)¹⁵C was measured using the new Texas Edinburgh Catania Silicon Array (TECSA). The next phase of the experimental program was to measure a reaction with a non-negligible interior contribution, for which ¹⁴C(d,p)¹⁵C at 60 MeV deuteron energy was used. This reaction turned out to be more peripheral than anticipated, and as a result, the ANC for the ground state was extracted from this measurement as well. The final results for the three measurements are C²2s1/2 = 1.96±0.16 fm⁻¹ for the ground state and C²1d5/2 = (4.23±0.38)·10⁻³ fm⁻¹ for the first excited state. Because the 60 MeV ¹⁴C(d,p)¹⁵C reaction turned out to have a very weak dependence on the interior, the SF could not be determined for the ¹⁴C+n ground state in ¹⁵C using the new method. A lower limit of 1.05 was found for the first excited state. It is possible that other reactions might turn out to be more suitable for this method, however, the difficulty encountered at this relatively high deuteron energy highlights a substantial problem likely to be seen in other applications. Using the ANCs determined in this work, the astrophysical ¹⁴C(n,γ)¹⁵C reaction rate was calculated. The resulting value for the cross section for capture to the ground state at 23 keV was σgs(23 keV)=5.1±0.4 μb and to the first excited state was σexc(23 keV)=0.2±0.02 μb.

Empirical likelihood (EL) is a recently developed nonparametric method of statistical inference. Owen’s 2001 book contains many important results for EL with uncensored data. However, fewer results are available for EL with right-censored data. In this dissertation, we first investigate a right-censored-data extension of Qin and Lawless (1994). They studied EL with uncensored data when the number of estimating equations is larger than the number of parameters (over-determined case). We obtain results similar to theirs for the maximum EL estimator and the EL ratio test, for the over-determined case, with right-censored data. We employ hazard-type constraints which are better able to handle right-censored data. Then we investigate EL with right-censored data and a k-sample mixed hazard-type constraint. We show that the EL ratio test statistic has a limiting chi-square distribution when k = 2. We also study the relationship between the constrained Kaplan-Meier estimator and the corresponding Nelson-Aalen estimator. We try to prove that they are asymptotically equivalent under certain conditions. Finally we present simulation studies and examples showing how to apply our theory and methodology with real data.

The thesis is based on the use of mathematical modeling and analysis to gain insightinto the transmission dynamics of malaria in a community. A new deterministic model for assessing the role of age-structure on the disease dynamics is designed. The model undergoes backward bifurcation, a dynamic phenomenon characterized by the co-existence of a stable disease-free and an endemic equilibrium of the model when the associated reproduction number is less than unity. It is shown that adding age-structure to the basic model for malaria transmission does not alter its essential qualitative dynamics. The study is extended to incorporate the use of anti-malaria drugs. Numerical simulations of the extended model suggest that for the case when treatment does not cause drug resistance (and the reproduction number of each of the two strains exceed unity), the model undergoes competitive exclusion. The impact of various effectiveness levels of the treatment strategy is assessed.

In this thesis, a method of distortion analysis of electromagnetic field sensors using orthogonal polynomial subspaces is presented. The effective height of the sensor is viewed as the impulse response of a linear system. The impulse response corresponds to a linear transformation which maps every electromagnetic incident field waveform to a received voltage waveform. Hermite and Laguerre orthogonal polynomials are used as the basis sets for the subspace of incident electromagnetic field waveforms. Using the selected basis set, a transformation matrix is calculated for the sensors. The transformation matrices are compared to a reference transformation matrix as a measure of distortion. The transformation matrices can describe the sensor behavior up to a certain frequency range. The limits on this frequency range are investigated for both Hermite-Gauss and Laguerre functions. The unique property of Laguerre functions is used to prove that the transformation matrix has a particular pattern. This method is applied on case studied sensors both in computer simulation and measurements.

Using the method of matched asymptotic expansions the reflection and transmission coefficients are calculated for scattering of oblique water waves by a vertical barrier. Here an assumption is made that the barrier is small compared to the wavelength and the depth of water. A number of sloshing problems are considered. The eigenfrequencies are calculated when a body is placed in a rectangular tank. Here the bodies considered are a vertical surface-piercing or bottom-mounted barrier, and circular and elliptic cylinders. When the body is a vertical barrier, the eigenfunction expansion method is applied. When the body is either a circular or elliptic cylinder, and the motion is two-dimensional, the boundary element method is applied to calculate the eigenfrequencies. For comparison, two approximations, "a wide-spacing", and "a small-body" are used for a vertical barrier and circular cylinder. In the wide-spacing approximation, the assumption is made that the wavelength is small compared with the distance between the body and walls. The small-body approximation means that a typical dimension of the body is much larger than the cross-sectional length scale of the fluid motion. For an elliptic cylinder, the method of matched asymptotic expansions is used and compared with the result of the boundary- element method. Also a higher-order solution is obtained using the method of matched asymptotic expansions, and it is compared with the exact solution for a surface-piercing barrier. Again the assumption is made that the length scale of the motion is much larger than a typical body dimension. Finally, the drift force on multiple bodies is considered the ratio of horizontal drift force in the direction of wave advance on two cylinders to that on an isolated cylinder is calculated. The method of matched asymptotic expansions is used under the assumption that the wavelength is much greater than the cylinder spacing.