Searcharxiv⌕ Search

arXiv subjects

Yukihide Takayama

Publications and source records attributed to Yukihide Takayama.

At least 19 recordsLinked to original sources
arXiv2019

Lifting iterated Frobenius over $W_n(k)$

Let $X$ be a smooth scheme over a perfect field $k$ of positive characteristic. M.V.~Nori and V.~Srinivas studied infinitesimal liftings of Frobenius morphism $F:X\to X$. Namely, given a Frobenius lifting $F_Y: Y\to Y$ over $W_{n-1}(k)$ the obstruction of lifting $F_Y$ over $W_n(k)$ is a class in $H^1(X, T_X\tensor B_1Ω^1_{X/k})$. We give a modified version of their theory and applications, in which we consider lifting $(n-1)$th iterated Frobenius $F^{n-1}:X\to X$ directly over $W_n(k)$.

math.AG↗
arXiv2017

On equivariant formal deformation theory

Using the set-up of deformation categories of Talpo and Vistoli, we re-interpret and generalize, in the context of cartesian morphisms in abstract categories, some results of Rim concerning obstructions against extensions of group actions in infinitesimal deformations. Furthermore, we observe that finite étale coverings can be infinitesimally extended and the resulting formal scheme is algebraizable. Finally, we show that pre-Tango structures survive under pullbacks with respect to finite, generically étale surjections $π:X\rightarrow Y$, and record some consequences regarding Kodaira vanishing in degree one.

math.AG↗
arXiv2017

Calabi-Yau threefolds in positive characteristic

In this note, an overview of Calabi-Yau varieties in positive characteristic is presented. Although Calabi-Yau varieties in characteristic zero are unobstructed, there are examples of Calabi-Yau threefolds in positive characteristic which cannot be lifted to characteristic zero, although one-dimensional and two-dimensional Calabi-Yau varieties, i.e., elliptic curves and K3 surfaces, are all liftable to characteristic zero. In this respect, Calabi-Yau threefolds in positive characteristic are interesting in view of deformation theory and they are still very mysterious.

math.AG↗
arXiv2014

On Kodaira type vanishing for Calabi-Yau threefolds in positive characteristic

We consider Calabi-Yau threefolds $X$ over an algebraically closed field $k$ of characteristic $p>0$ that are not liftable to characteristic $0$ or liftable ones with $p=2$. It is unknown whether Kodaira vanishing holds for these varieties. In this paper, we give a lower bound of $h^1(X, L^{-1})=\dim_k H^1(X, L^{-1})$ if $L$ is an ample divisor with $H^1(X, L^{-1})\ne{0}$. Moreover, we show that a Kodaira type vanishing holds if $X$ is a Schröer variety or a Schoen variety, which extends the similar result given in my previous paper for the Hirokado variety. We show that such kind of vanishing holds for Calabi-Yau manifold whose Picard variety has no $p$-torsion. Also we show that a modified Raynaud-Mukai construction does not produce any counter-example to Kodaira vanishing.

math.AG↗
arXiv2012

Kodaira type vanishing theorem for the Hirokado variety

The Hirokado variety is a Calabi-Yau threefold in characteristic 3 that is not liftable either to characteristic~0 or the ring $W_2$ of the second Witt vectors. Although Deligne-Illusie-Raynaud type Kodaira vanishing cannot be applied, we show that $H^1(X, L^{-1})=0$, for an ample line bundle such that $L^3$ has a non-trivial global section, holds for this variety.

math.AG↗
arXiv2011

Raynaud-Mukai construction and Calabi-Yau Threefolds in Positive Characteristic

In this article, we study the possibility of producing a Calabi-Yau threefold in positive characteristic which is a counter-example to Kodaira vanishing. The only known method to construct the counter-example is so called inductive method such as the Raynaud-Mukai construction or Russel construction. We consider Mukai's method and its modification. Finally, as an application of Shepherd-Barron vanishing theorem of Fano threefolds, we compute $H^1(X, H^{-1})$ for any ample line bundle $H$ on a Calabi-Yau threefold $X$ on which Kodaira vanishing fails.

math.AG↗
arXiv2009

On non-vanishing of cohomologies of generalized Raynaud polarized surfaces

We consider a family of slightly extended version of the Raynaud's surfaces X over the field of positive characteristic with Mumford-Szpiro type polarizations Z, which have Kodaira non-vanishing H^1(X, Z^{-1})\ne 0. The surfaces are at least normal but smooth under a special condition. We compute the cohomologies H^i(X, Z^n), for intergers i and n, and study their (non-)vanishing. Finally, we give a fairly large family of non Mumford-Szpiro type polarizations Z_{a,b} with Kodaira non-vanishing.

math.AG↗
arXiv2006

Local cohomologies of isolated non F-rational singularities

In this paper, we consider positively graded isolated non F-rational singularities (R,m) with d=dim R over the field K of positive characteristic. We give a representation of lower local cohomologies H^i_m(R) (i<d) in terms of tight closure and limit closure of certain type of parameters. As an application to isolated singularities, we show a relation between non-vanishing of the tight closure of zero in the highest local cohomology (0)^*_{H^d(R)} and non-vanishing of the cohomology H^{d-1}_m(R).

math.AC↗
arXiv2005

Combinatorial characterizations of generalized Cohen-Macaulay monomial ideals

We give a generalization of Hochster's formula for local cohomologies of square-free monomial ideals to monomial ideals, which are not necessarily square-free. Using this formula, we give combinatorial characterizations of generalized Cohen-Macaulay monomial ideals. We also give other applications of the generalized Hochster's formula.

math.AC↗
arXiv2004

A generalized Hochster's formula for local cohomologies of monomial ideals

The Hilbert series of local cohomologies for monomial ideals, which are not necessarily square-free, is established. As applications, we give a sharp lower bound of the non-vanishing degree of local cohomologies and also a sharp lower bound of the positive integer k of k-Buchsbaumness for generalized Cohen-Macaulay monomial ideals.

math.AC↗
arXiv2004

Ideals whose associated graded rings are isomorphic to the base rings

Let $k$ be a field. We determine the ideals $I$ in a finitely generated graded $k$-algebra $A$, whose associated graded rings are isomorphic to $A$. Also we compute the graded local cohomologies of the Rees rings $A[I t]$ and give the condition for $A[I t]$ to be generalized Cohen-Macaulay.

math.AC↗
arXiv2003

B-sequence and approximations of generalized Cohen-Macaulay ideals

We introduce the notion of b-sequence for finitely generated modules over Noetherian rings, which characterizes long Bourbaki sequences. Our main concern is an application of this notion to generalized Cohen-Macaulay approximation, which we introduced lately. We will show how we can construct long Bourbaki sequences of non-trivial type characterizing generalized Cohen-Macaualy ideals by finding suitable b-sequences.

math.AC↗