An application of the almost purity theorem to the homological conjectures

The aim of this article is to establish the existence of big Cohen-Macaulay algebras in mixed characteristic in some special situation. The main result follows from the so-called almost purity theorem proved by Davis and Kedlaya.Comment: to appear in Journal of Pure and Applied Algebr

Almost Cohen-Macaulay algebras in mixed characteristic via Fontaine rings

In the present paper, it is proved that any complete local domain of mixed characteristic has a weakly almost Cohen-Macaulay algebra in the sense that some system of parameters is a weakly almost regular sequence, which is a notion defined via a valuation. The central idea of this result originates from the main statement obtained by Heitmann to prove the Monomial Conjecture in dimension 3. In fact, A weakly almost Cohen-Macaulay algebra is constructed over the absolute integral closure of a complete local domain by applying the methods of Fontaine rings and Witt vectors. A connection of the main theorem with the Monomial Conjecture is also discussed.Comment: To appear in Illinois J. of Mat

The Frobenius action on local cohomology modules in mixed characteristic

R. Heitmann's proof of the Direct Summand Conjecture has opened a new approach to the study of homological conjectures in mixed characteristic. Inspired by his work and by the methods of almost ring theory, we discuss a normalized length for certain torsion modules, which was introduced by G. Faltings. Using the normalized length and the Frobenius map, we prove some results of local cohomology for local rings in mixed characteristic, which has an immediate implication for the subject of splinters studied by A. Singh.Comment: 15 pages, to appear in Compositio Mat

Integral perfectoid big Cohen-Macaulay algebras via Andr\'e's theorem

The main result of this article is to prove that any Noetherian local domain of mixed characteristic maps to an integral perfectoid big Cohen-Macaulay algebra. The proof of this result is based on the construction of almost Cohen-Macaulay algebras in mixed characteristic due to Yves Andr\'e. Moreover, we prove that the absolute integral closure of a complete Noetherian local domain of mixed characteristic maps to an integral perfectoid big Cohen-Macaulay algebra.Comment: Final version: to appear in Math. Annale

An embedding problem of Noetherian rings into the Witt vectors

The aim of this article is to prove some results on the existence of an integral extension domain of a complete local Noetherian domain in mixed characteristic $p>0$ having certain distinguished properties with respect to the Frobenius map. We prove the main results by constructing required extension domains via Witt vectors and the method of maximal \'etale extensions. It is worth remarking that the resulting algebras have deep connections with the homological conjectures and the rings in $p$-adic Hodge theory.Comment: minor change

F-coherent rings with applications to tight closure theory

The aim of this paper is to introduce a new class of Noetherian rings of positive characteristic in terms of perfect closures and study their basic properties. If the perfect closure of a Noetherian ring is coherent, we call it an $F$-coherent ring. Some interesting applications are given in connection with tight closure theory. In particular, we discuss relationships between $F$-coherent rings and $F$-pure, $F$-regular, and $F$-injective rings. The final section discusses how the coherent property effects the behavior of tight closure for general perfect rings.Comment: 10 pages, comments are welcom

On the Witt vectors of perfect rings in positive characteristic

The purpose of this article is to prove some results on the Witt vectors of perfect $\mathbf{F}_p$-algebras. Let $A$ be a perfect $\mathbf{F}_p$-algebra for a prime integer $p$ and assume that $A$ has the property $\mathbf{P}$. Then does the ring of Witt vectors of $A$ also have $\mathbf{P}$? A main theorem gives an affirmative answer for \mathbf{P}="\mbox{integrally closed}" under a very mild condition.Comment: to appear in Communications in Algebr

Almost Cohen-Macaulay and almost regular algebras via almost flat extensions

The present paper deals with various aspects of the notion of almost Cohen-Macaulay property, which was introduced and studied by Roberts, Singh and Srinivas. We employ the definition of almost zero modules as defined by a value map, which is different from the version of Gabber-Ramero. We prove that, if the local cohomology modules of an algebra $T$ of certain type over a local Noetherian ring are almost zero, $T$ maps to a big Cohen-Macaulay algebra. Then we study how the almost Cohen-Macaulay property behaves under almost faithfully flat extension. As a consequence, we study the structure of $F$-coherent rings of positive characteristic in terms of almost regularity.Comment: to appear in J. Commutative Algebr

Specialization Method in Krull Dimension two and Euler System Theory over Normal Deformation Rings

The aim of this article is to establish the specialization method on characteristic ideals for finitely generated torsion modules over a complete local normal domain R that is module-finite over $O[[x_1, ..., x_d]]$, where $O$ is the ring of integers of a finite extension of the field of p-adic integers $Q_p$. The specialization method is a technique that recovers the information on the characteristic ideal $char_R(M)$ from $char_{R/I}(M/IM)$, where I varies in a certain family of nonzero principal ideals of R. As applications, we prove Euler system bound over Cohen-Macaulay normal domains by combining the main results in an earlier article of the first named author and then we prove one of divisibilities of the Iwasawa main conjecture for two-variable Hida deformations generalizing the main theorem obtained in an article of the first named author

An elementary proof of Cohen-Gabber theorem in the equal characteristic p>0p>0 case

The aim of this article is to give a new proof of Cohen-Gabber theorem in the equal characteristic $p>0$ case.Comment: 13 pages, to appear in Tohoku Math.