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Search Results: 1 - 10 of 4293 matches for " Kazuhiro Yamamoto "
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Path Integral Formulation for Wave Effect in Multilens System  [PDF]
Kazuhiro Yamamoto
International Journal of Astronomy and Astrophysics (IJAA) , 2017, DOI: 10.4236/ijaa.2017.73018
Abstract: A formula to investigate the wave effect in a multi-lens system is presented on the basis of a path integral formalism by generalizing the work by Nakamura and Deguchi (1999). The wave effect of a system with two lenses is investigated in an analytic way as a simple application to demonstrate usefulness of the formula and variety of wave effect in multi-lens system.
Effects of Microstructural Modification Using Friction Stir Processing on Fatigue Strength of Butt-Welded Joints for High-Strength Steels  [PDF]
Hajime Yamamoto, Kazuhiro Ito
Materials Sciences and Applications (MSA) , 2018, DOI: 10.4236/msa.2018.97045
Abstract: Friction stir processing (FSP) is an effective surface-microstructure modification technique using a rotational tool to refine and homogenize microstructure of metallic materials. In this study, FSP was conducted on the surface of the heat-affected zone (HAZ), which is a region exhibiting degraded mechanical properties and shown to have microstructural changes, of butt-welded joints for two high-strength steels with tensile strength grades of 490 MPa and 780 MPa (hereafter HT490 and HT780, respectively). Inhomogeneous mixing of materials derived from weld metals and base metals (BMs) in a stir zone (SZ) produced inhomogeneous distribution of elements and microstructure depending on the set of the advancing side and retreating side in the SZs. The welded joints with FSP for HT490 exhibited higher hardness than that of the BM through whole of the SZ surface (fine polygonal ferrite grains and bainite structure with laths at the Mn-rich and Mn-poor regions, respectively). On the other hand, those for HT780 exhibited the minimum hardness value similar to that of the BM at the SZ surface (a few polygonal ferrite grains in the matrix of martensite laths). Fatigue strength increased by about 35 MPa and 15 MPa in stress amplitude at 107 cycles as fatigue limit due to FSP. Fatigue failure occurred at the BM and the SZ, respectively, in the welded joints modified by FSP for HT490 and HT780, in comparison with the HAZs in the as-welded joints for both grade steels. The difference in fatigue strength increase due to FSP and failure location between the welded joints for HT490 and HT780 can be attributed to the topmost SZ microstructures and their distribution.
Measuring Cosmological Parameters with the SDSS QSO Spatial Power Spectrum Analysis to Test the Cosmological Principle
Kazuhiro Yamamoto
Physics , 2003, DOI: 10.1046/j.1365-8711.2003.06477.x
Abstract: In this paper we emphasize the importance of the Sloan Digital Sky Survey (SDSS) QSO clustering statistics as a unique probe of the Universe. Because the complete SDSS QSO sample covers a quarter of the observable universe, cosmological parameters estimated from the clustering statistics have an implication as a test of the cosmological principle, by comparing with those from local galaxies and other cosmological observations. Using an analytic approach to the power spectrum for the QSO sample, we assess the accuracy with which the cosmological parameters can be determined. Arguments based on the Fisher matrix approach demonstrate that the SDSS QSO sample might have a potential to provide useful constraints on the density parameters as well as the cosmic equation of state.
Optimal Weighting Scheme in Redshift-space Power Spectrum Analysis and a Prospect for Measuring the Cosmic Equation of State
Kazuhiro Yamamoto
Physics , 2002, DOI: 10.1086/377488
Abstract: We develop a useful formula for power spectrum analysis for high and intermediate redshift galaxy samples, as an extension of the work by Feldman, Kaiser & Peacock (1994). An optimal weight factor, which minimizes the errors of the power spectrum estimator, is obtained so that the light-cone effect and redshift-space distortions are incorporated. Using this formula, we assess the feasibility of the power spectrum analysis with the luminous red galaxy (LRG) sample in the Sloan Digital Sky Survey as a probe of the equation of state of the dark energy. Fisher matrix analysis shows that the LRG sample can be sensitive to the equation of state around redshift z=0.13. It is also demonstrated that the LRG sample can constrain the equation of state with (1-sigma) error of 10% level, if other fundamental cosmological parameters are well determined independently. For the useful constraint, we point out the importance of modeling the bias taking the luminosity dependence into account. We also discuss the optimized strategy to constrain the equation of state using power spectrum analysis. For a sample with fixed total number of objects, it is most advantageous to have the sample with the mean number density $10^{-4} h^3{\rm Mpc}^{-3}$ in the range of the redshift $0.4 \simlt z\simlt 1$.
Cosmological constraint from QSO spatial power spectrum
Kazuhiro Yamamoto
Physics , 2001, DOI: 10.1007/10856495_86
Abstract: In this paper we consider constraints on the cosmological density parameters from the spatial power spectrum of QSOs. We first review an analytic approach to the spatial power spectrum of QSOs, then we compare the result of the analytic approach with a preliminary result of the power spectrum from the two-degree Field QSO redshift (2QZ) survey. From a simple chi^2 test, we show that a finite baryon fraction better explains observation of the QSO power spectrum, which might suggest a possible detection of the baryonic oscillations in the QSO power spectrum.
Covariant Wigner Function Approach to the Boltzmann Equation for Mixing Fermions in Curved Spacetime
Kazuhiro Yamamoto
Physics , 2003, DOI: 10.1142/S0217751X03015295
Abstract: Based on the covariant Wigner function approach we derive the quantum Boltzmann equation for fermions with flavor mixing in general curved spacetime. This work gives a rigorous theoretical framework to investigate the flavor oscillation phenomena taking the gravitational effect into account. It is shown that the Boltzmann equation of the lowest order of the expansion with respect to $\hbar$ reproduces the previous result which was derived in the relativistic limit on the Minkowski background spacetime. It is demonstrated that the familiar formula for the vacuum neutrino oscillation can be obtained by solving the Boltzmann equation. Higher order effects of the $\hbar$-expansion is also briefly discussed.
Cosmological Constraint from the 2dF QSO Spatial Power Spectrum
Kazuhiro Yamamoto
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05575.x
Abstract: In this paper we obtain constraints on the cosmological density parameters, \Omega_m and \Omega_b, by comparing the preliminary measurement of the QSO power spectrum from the two degree field QSO redshift survey with results from an analytic technique of power spectrum estimation, described in this paper. We demonstrate the validity of the analytic approach by comparing the results with the power spectrum of an N-body simulation. We find a better fit to the shape of the QSO power spectrum for low density models with \Omega_m= 0.1-0.4. We show that a finite baryon fraction \Omega_b/\Omega_m=0.2, consistent with observations of the CMB anisotropies and nucleosynthesis, fits the observational result of the 2QZ survey better, though the constraint is not very tight. By using the Fisher matrix technique, we investigate just how a survey would be required before a significant constraint on the density parameters can be made. We demonstrate that the constraint could be significantly improved if the survey was four times larger.
Analyses of the Anatomy of the Ureter near Ureterovesical Junction on the Unenhanced CT  [PDF]
Hiroshi Juri, Kazuhiro Yamamoto, Yoshifumi Narumi
Open Journal of Radiology (OJRad) , 2016, DOI: 10.4236/ojrad.2016.62012
Abstract: Purpose: There has been no report evaluating the anatomy of the ureter on CT images. The purpose of this study was to retrospectively evaluate the anatomy of the ureter near the ureterovesical junction on preoperative CT. Methods: Forty patients were analyzed the craniocaudal level of the ureterovesical junction, the transverse distance from the medial line of the body to the ureterovesical junction, the direction of the ureter from the ureterovesical junction, and the length of the ureter from the point of turning medially to the ureterovesical junction. Results: The ureterovesical junction was almost at the level of the upper or middle of the left femoral head. The mean transverse distance on the right and left side was 19.0 and 19.3 mm. The direction of the ureter was almost posterior, lateral, and cranial, but had a wide range of angles. The mean length of the ureter on the right and left side was 24.0 and 23.9 mm. Conclusions: We can evaluate the anatomy of the ureter near the ureterovesical junction on preoperative CT.
Characterizing the linear growth rate of cosmological density perturbations in an f(R) model
Tatsuya Narikawa,Kazuhiro Yamamoto
Physics , 2009, DOI: 10.1103/PhysRevD.81.129903
Abstract: We investigate the linear growth rate of cosmological matter density perturbations of a viable f(R) model both numerically and analytically. We find that the growth rate in the scalar-tensor regime can be characterized by a simple analytic formula. We also investigate a prospect of constraining the Compton wavelength scale of the f(R) model with a future weak lensing survey.
Two-point correlation function of high-redshift objects: an explicit formulation on a light-cone hypersurface
Kazuhiro Yamamoto,Yasushi Suto
Physics , 1999, DOI: 10.1086/307184
Abstract: While all the cosmological observations are carried out on a light-cone, the null hypersurface of an observer at z=0, the clustering statistics has been properly defined only on the constant-time hypersurface. We develop a theoretical formulation for a two-point correlation function on the light-cone, and derive a practical approximate expression relevant to the discussion of clustering of high-redshift objects at large separations. As a specific example, we present predictions of the two-point correlation function for the Durham/AAT, SDSS and 2dF quasar catalogues. We also briefly discuss the effects of adopted luminosity function, cosmological parameters and bias models on the correlation function on the light-cone.
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