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Masahiro Ohno

Publications and source records attributed to Masahiro Ohno.

9 recordsLinked to original sources
arXiv2018

Nef vector bundles on a projective space with first Chern class three

We classify nef vector bundles on a projective space with first Chern class three over an algebraically closed field of characteristic zero; we see, in particular, that these nef vector bundles are globally generated if the second Chern class is less than eight, and that there exist nef but non-globally generated vector bundles with second Chern class eight and nine on a projective plane.

math.AG↗
arXiv2014

Spaces of algebraic maps from real projective spaces to toric varieties

The problem of approximating the infinite dimensional space of all continuous maps from an algebraic variety $X$ to an algebraic variety $Y$ by finite dimensional spaces of algebraic maps arises in several areas of geometry and mathematical physics. An often considered formulation of the problem (sometimes called the Atiyah-Jones problem after \cite{AJ}) is to determine a (preferably optimal) integer $n_D$ such that the inclusion from this finite dimensional algebraic space into the corresponding infinite dimensional one induces isomorphisms of homology (or homotopy) groups through dimension $n_D$, where $D$ denotes a tuple of integers called the "degree" of the algebraic maps and $n_D\to\infty$ as $D\to\infty$. In this paper we investigate this problem in the case when $X$ is a real projective space and $Y$ is a smooth compact toric variety.

math.AT↗
arXiv2011

Spectral Evolutions in Gamma-Ray Burst Exponential Decays Observed with Suzaku WAM

This paper presents a study on the spectral evolution of gamma-ray burst (GRB) prompt emissions observed with the Suzaku Wide-band All-sky Monitor (WAM). By making use of the WAM data archive, 6 bright GRBs exhibiting 7 well-separated fast-rise-exponential-decay (FRED) shaped light curves are presented and the evaluated exponential decay time constants of the energy-resolved light curves from these FRED peak light curves are shown to indicate significant spectral evolution. The energy dependence of the time constants is well described with a power-law function tau(E) ~ E^gamma, where gamma ~ -(0.34 +/- 0.12) in average, although 5 FRED peaks show consistent value of gamma = -1/2 which is expected in synchrotron or inverse-Compton cooling models. In particular, 2 of the GRBs were located with accuracy sufficient to evaluate the time-resolved spectra with precise energy response matrices. Their behavior in spectral evolution suggests two different origins of emissions. In the case of GRB081224, the derived 1-s time-resolved spectra are well described by a blackbody radiation model with a power-law component. The derived behavior of cooling is consistent with that expected from radiative cooling or expansion of the emission region. On the other hand, the other 1-s time-resolved spectra from GRB100707A is well described by a Band GRB model as well as with the thermal model. Although relative poor statistics prevent us to conclude, the energy dependence in decaying light curve is consistent with that expected in the former emission mechanism model.

astro-ph.HE↗
arXiv2005

Classification of generalized polarized manifolds by their nef values

Let (M,E) be a generalized polarized manifold, i.e., a pair of an n-dimensional smooth projective variety and an ample vector bundle E of rank r on M. Let t be the nef value of a polarized manifold (M, det E), i.e., the minimum of the set of real numbers s such that K+s det E is nef; we have tr<= n+1 by Mori's theory. In this paper we classify the pairs (M,E) in the following two cases: (1) n-2<= tr and t >= 1; (2) n+1-tr< t <= 1.

math.AG↗