Unmixed local rings with minimal Hilbert-Kunz multiplicity are regular
Craig Huneke, Yongwei Yao
给出了Hilbert-Kunz重数为1的非混合局部环必正则的一个新的简单证明.
We give a new and simple proof that unmixed local rings having Hilbert-Kunz multiplicity equal to 1 must be regular.
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In this paper we give new lower bounds on the Hilbert-Kunz multiplicity of unmixed non-regular local rings, bounding them uniformly away from one. Our results improve previous work of Aberbach and Enescu.
On rings with small Hilbert-Kunz multiplicity
Watanabe和Yoshida的一个结果是：正特征的非混合局部环是正则的当且仅当它的Hilbert-Kunz重数为1.我们证明了，对于固定的$p$（特征）和$d$（维数），存在一个数$ε(d,p) > 0$使得任何非正则非混合环$R$至少有$1+ε(d,p)$的Hilbert-Kunz重数.我们还证明了具有足够小的Hilbert-Kunz重数的局部环是Cohen-Macaulay和$F$ -有理数的.
Notes on Hilbert-Kunz multiplicity of Rees algebras
本文利用基环的一些不变量估计了（扩张）Rees代数的Hilbert-Kunz重数。同时，我们给出了Veronese子环上Rees代数的Hilbert-Kunz重数的一个显式公式。
We prove that the Hilbert-Kunz multiplicity is upper semi-continuous in F-finite rings and algebras of essentially finite type over an excellent local ring.
We define a function, called s-multiplicity, that interpolates between Hilbert-Samuel multiplicity and Hilbert-Kunz multiplicity by comparing powers of ideals to the Frobenius powers of ideals. The function is continuous in s, and its value is equal to Hilbert-Samuel multiplicity for small values of s and is equal to Hilbert-Kunz multiplicity for large values of s. We prove that it has an Associativity Formula generalizing the Associativity Formulas for Hilbert-Samuel and Hilbert-Kunz multiplicity. We also define a family of closures such that if two ideals have the same s-closure then they ha…
Generalized Hilbert-Kunz Multiplicity for Families of Ideals
本文系统研究正特征Noetherian局部环中理想族的广义Hilbert-Kunz重数，引入Amao型重数这一新渐近不变量，证明p-理想族的广义Hilbert-Kunz重数是Amao型重数的极限。
On the upper semi-continuity of the Hilbert-Kunz multiplicity
我们证明了$d$维非正则完全交在$F_p$的代数闭包上的Hilbert-Kunz重数是由超曲面$\sum _{i=0}^{d} x_i^2=0$的Hilbert-Kunz重数下有界的，在完全交的情况下，肯定地回答了Watanabe和Yoshida的一个猜想。
Hilbert-Kunz multiplicity of powers of ideals in dimension two
本文研究二维局部环中理想幂的Hilbert-Kunz重数，解答了Smirnov的问题，并利用Ratliff-Rush版本给出判定准则。
Let $R$ be a local ring of characteristic $p>0$ which is $F$-finite and has perfect residue field. We compute the generalized Hilbert-Kunz invariant for certain modules over several classes of rings: hypersurfaces of finite representation type, toric rings, $F$-regular rings.
Hilbert-Kunz Multiplicity of Fiber Product Rings and Nagata Idealizations
本文为纤维积环和Nagata理想化推导出Hilbert-Kunz重数的显式公式，并给出新界与结构结果。
We establish the continuity of Hilbert-Kunz multiplicity and F-signature as functions from a Cohen-Macaulay local ring $(R,\m,k)$ of prime characteristic to the real numbers at reduced parameter elements with respect to the $\m$-adic topology.
We give bounds for the Hilbert-Kunz multiplicity of the product of two ideals, and we characterize the equality in terms of the tight closures of the ideals. Connections are drawn with $*$-spread and with ordinary length calculations.
Lower bounds for Hilbert-Kunz multiplicities in local rings of fixed dimension
设$(R,\m)$是具有正素特征和维数$d$的形式非混合局部环.我们研究了具有小希尔伯特-昆兹多重性的含义（即，接近1）。特别是，我们表明，如果$R$是不规则的，存在一个下界，严格大于1，仅取决于$d$，其希尔伯特-昆兹多重性。
We prove that the Hilbert-Kunz function of the ideal $(I,It)$ of the Rees algebra $\mathcal{R}(I)$, where $I$ is an $\mathfrak{m}$-primary ideal of a $1$-dimensional local ring $(R,\mathfrak{m})$, is a quasi-polynomial in $e$, for large $e.$ For $s \in \mathbb{N}$, we calculate the Hilbert-Samuel function of the $R$-module $I^{[s]}$ and obtain an explicit description of the generalized Hilbert-Kunz function of the ideal $(I,It)\mathcal{R}(I)$ when $I$ is a parameter ideal in a Cohen-Macaulay local ring of dimension $d \geq 2$, proving that the generalized Hilbert-Kunz function is a piecewise p…
We present results on the Watanabe-Yoshida conjecture for the Hilbert-Kunz multiplicity of a local ring of positive characteristic. By improving on a "volume estimate" giving a lower bound for Hilbert-Kunz multiplicity, we obtain the conjecture when the ring either has Hilbert-Samuel multiplicity less than or equal to five, or dimension less than or equal to six. For non-regular rings with fixed dimension, a new lower bound for the Hilbert-Kunz multiplicity is obtained.
Minimal Hilbert-Kunz multiplicity
在本文中，我们问以下问题：什么是理想$I' \supseteq I$与$l_A(I'/I) =1$的差$e_{HK}(I) - e_{HK}(I')$的极小值？为了回答这个问题，我们定义了强F-正则环的极小Hilbert-Kunz重数的概念。此外，我们计算这个不变量的商奇点和Segre嵌入的坐标环：$P^{r-1} \times P^{s-1} \hookrightarrow P^{rs-1}$，分别。
We develop Hilbert-Kunz theory in a combinatorial setting namely for binoids. We show that the Hilbert-Kunz multiplicity for commutative, finitely generated, semipositive, cancellative and reduced binoids exists and is a rational number. This implies that the corresponding Hilbert-Kunz multiplicity for the binoid algebras does not depend on the characteristic.
In this note we first give a new bound on $e_{HK}(\sim)$ the Hilbert-Kunz multiplicity of invariant rings, by the help of the Noether's bound. Then, we simplify, extend and present applications of the reciprocity formulae due to L. Smith. He proved the formula over polynomial rings and his result is tight in the following sense: Over complete intersection rings with isolated singularity we show that the reciprocity formulae "$e_{HK}(R/I)+ e_{HK}(R/J) = e_{HK}(R/\underline{f})$" is equivalent with $\pd(I)<\infty$ when $I$ is an $\fm$-primary unmixed ideal linked to $J$ along with a regular sequ…
We study Hilbert-Kunz multiplicity of the powers of an ideal and establish existence of the second coefficient at the full level of generality, thus extending a recent result of Trivedi. We describe the second coefficient as the limit of the Hilbert coefficients of Frobenius powers and show that it is additive in short exact sequences and satisfies a Northcott-type inequality.
Hilbert-Kunz multiplicity of three-dimensional local rings
本文研究了具有特征$p>0$的克鲁尔维数非正则非混合局部环的Hilbert-Kunz重数的一个下界（即$s_{HK}(p,d)$）.特别地，我们关注三维局部环。实际上，作为主要结果，我们将证明在较弱的条件下，$s_{HK}(p,3) = 4/3$和Hilbert-Kunz重数为4/3的三维完全局部环同构于非退化二次超平面$k[[X,Y,Z,W]]/(X^2+Y^2+Z^2+W^2)$.此外，我们提出了一个推广的主要定理的情况下，$\dim A \ge 4$作为一个猜想，并表明这也是真实的情况下，$\dim A = 4$使用类似的方法，在证明的主要定理。
Hilbert-Kunz multiplicity of quadrics decreases
通过证明Z/p^eZ的Green环与Z/pZ的Green环的e重张量积同构，并利用p-adic展开，计算Fermat二次曲面的Hilbert-Kunz函数和重数，证明其随特征递减，肯定Yoshida猜想。
Let $R$ be an excellent Noetherian ring of prime characteristic. Consider an arbitrary nested pair of ideals (or more generally, a nested pair of submodules of a fixed finite module). We do \emph{not} assume that their quotient has finite length. In this paper, we develop various sufficient numerical criteria for when the tight closures of these ideals (or submodules) match. For some of the criteria we only prove sufficiency, while some are shown to be equivalent to the tight closures matching. We compare the various numerical measures (in some cases demonstrating that the different measures g…
We prove in a broad combinatorial setting, namely for finitely generated semipositive cancellative reduced binoids, that the Hilbert-Kunz multiplicity is a rational number independent of the characteristic.
Hilbert-Kunz multiplicity and reduction mod p
我们证明了特征为0的不可约射影曲线的正特征约化的Hilbert-Kunz重数在特征趋于无穷大时有一个明确的极限。
We show that Bertini theorems hold for $F$-signature and Hilbert--Kunz multiplicity. In particular, if $X \subseteq \mathbb{P}^n$ is normal and quasi-projective with $F$-signature greater than $λ$ (respectively the Hilbert--Kunz multiplicity is less than $λ$) at all points $x \in X$, then for a general hyperplane $H \subseteq \mathbb{P}^n$ the $F$-signature (respectively Hilbert--Kunz multiplicity) of $X \cap H$ is greater than $λ$ (respectively less than $λ$) at all points $x \in X \cap H$.
The rationality of the Hilbert-Kunz multiplicity in graded dimension two
证明了对于R_+-准素齐次理想I=（f_1，. f_n）在正特征的代数闭域上的有限型二维分次域R中的.此外，我们还给出了合冲丛Syz（f_1，.）的强Harder-Narasimhan滤的某些有理数的Hilbert-Kunz重数公式。. ., f_n）在射影曲线Y = Proj R上的投影。
For a standard graded ring $R$ of dimension $\geq 2$ over a perfect field of characteristic $p>0$ and a homogeneous ideal $I$ of finite colength, the HK density function of $R$ with respect to $I$ is a compactly supported continuous function $f_{R, I}:[0, \infty)\longto [0, \infty)$, whose integration yields the \mbox{HK} multiplicity $e_{HK}(R, I)$. Here we answer a question of V. Trivedi about the Hilbert-Kunz density function of the tensor product of standard graded rings and show that it is the convolution of the Hilbert-Kunz density function of the factor rings. Using Fourier transform, a…
Bounds for the Hilbert-Kunz Multiplicity of Singular Rings
本文通过引入插值函数$\varphi_J$并证明其凹性，结合已知技术得到改进下界，从而证明了Watanabe-Yoshida猜想在维数至多$7$时成立。
This is a survey article on Hilbert-Kunz multiplicity and the F-signature.
We prove that the generalized Hilbert-Kunz function of a graded module $M$ over a two-dimensional standard graded normal $K$-domain over an algebraically closed field $K$ of prime characteristic $p$ has the form $gHK(M,q)=e_{gHK}(M)q^{2}+γ(q)$, with rational generalized Hilbert-Kunz multiplicity $e_{gHK}(M)$ and a bounded function $γ(q)$. Moreover we prove that if $R$ is a $\mathbb{Z}$-algebra, the limit for $p\rightarrow+\infty$ of the generalized Hilbert-Kunz multiplicity $e_{gHK}^{R_p}(M_p)$ over the fibers $R_p$ exists and it is a rational number.
Let p be a prime. The Hilbert-Kunz multiplicity, mu, of the element sum(x_i^(d_i)) of (Z/p)[x_1,..., x_s] depends on p in a complicated way. We calculate the limit of mu as p -> infinity. In particular when each d_i is 2 we show that the limit is 1 + the coefficient of z^(s-1) in the power series expansion of sec z + tan z.
Let $k$ be an algebraically closed field of characteristic $p > 0$. We show that if $X\subseteq\mathbb{P}^n_k$ is an equidimensional subscheme with Hilbert--Kunz multiplicity less than $λ$ at all points $x\in X$, then for a general hyperplane $H\subseteq\mathbb{P}^n_k$, the Hilbert--Kunz multiplicity of $X\cap H$ is less than $λ$ at all points $x\in X\cap H$. This answers a conjecture and generalizes a result of Carvajal-Rojas, Schwede and Tucker, whose conclusion is the same as ours when $X\subseteq\mathbb{P}^n_k$ is normal. In the process, we substantially generalize certain uniform estimate…
This paper develops a theory of equimultiplicity for Hilbert-Kunz multiplicity and uses it to study the behavior of Hilbert-Kunz multiplicity on the Brenner-Monsky hypersurface. A number of applications follows, in particular we show that Hilbert-Kunz multiplicity attains infinitely many values and that equimultiple strata may not be locally closed.
This paper concerns the question of whether a more direct limit can be used to obtain the limit Hilbert-Kunz multiplicity, a possible candidate for a characteristic zero Hilbert-Kunz multiplicity. The main goal is to establish an affirmative answer for one of the main cases for which the limit Hilbert-Kunz multiplicity is even known to exist, namely that of graded ideals in the homogeneous coordinate ring of smooth projective curves. The proof involves more careful estimates of bounds found independently by Brenner and Trivedi on the dimensions of the cohomologies of twists of the syzygy bundl…
Hilbert-Kunz multiplicity of quadrics via Ehrhart theory
本文通过Ehrhart多项式研究二次曲面的Hilbert-Kunz乘数，证明其为特征p的有理函数，并揭示其随维度变化的性质。
The Hilbert-Kunz multiplicity and $F$-signature are important invariants for researchers in commutative algebra and algebraic geometry. We provide software, and describe the automation of a calculation, for the two invariants in the case of intersection algebras over polynomial rings.
In this paper we define and study the global Hilbert-Kunz multiplicity and the global F-signature of prime characteristic rings which are not necessarily local. Our techniques are made meaningful by extending many known theorems about Hilbert-Kunz multiplicity and F-signature to the non-local case.
We interpret Hilbert-Kunz theory of a graded ring of positive characteristic in terms of Frobenius asymptotic of cohomology of vector bundles on projective varieties. With this method we show that for almost all prime numbers there exist three-dimensional quartic hypersurface domains and modules of finite length with irrational Hilbert-Kunz multiplicity. From this we deduce that also the Hilbert-Kunz multiplicity of a local noetherian domain might be an irrational number.
For a pair $(M, I)$, where $M$ is finitely generated graded module over a standard graded ring $R$ of dimension $d$, and $I$ is a graded ideal with $\ell(R/I) < \infty$, we introduce a new invariant $HKd(M, I)$ called the {\em Hilbert-Kunz density function}. In Theorem 1.1, we relate this to the Hilbert-Kunz multiplicity $e_{HK}(M,I)$ by an integral formula. We prove that the Hilbert-Kunz density function is additive. Moreover it satisfies a multiplicative formula for a Segre product of rings. This gives a formula for $e_{HK}$ of the Segre product of rings in terms of the HKd of the rings invo…
Hilbert-Kunz multiplicity and F-signature are numerical invariants of commutative rings in positive characteristic that measure severity of singularities: for a regular ring both invariants are equal to one and the converse holds under mild assumptions. A natural question is for what singular rings these invariants are closest to one. For Hilbert--Kunz multiplicity this question was first considered by the last two authors and attracted significant attention. In this paper, we study this question, i.e., an upper bound, for F-signature and revisit lower bounds on Hilbert--Kunz multiplicity.
A Hilbert-Kunz criterion for solid closure in dimension two (characteristic zero)
设I表示特征为零的代数闭域上二维正规标准分次域中的齐次R_+准素理想。证明了齐次元f属于实闭包I^* 当且仅当e_{HK}（I）= e_{HK}（（I，f）），其中e_{HK}表示理想的（特征零）Hilbert-Kunz重数.这提供了一个版本的特征零的著名的希尔伯特-昆兹准则紧封闭的积极的特点。
In this article, I give an iterative closed form formula for the Hilbert-Kunz function for any binomial hypersurface in general, over any feild of arbitrary positive characteristic. I prove that the Hilbert-Kunz multiplicity associated to any Binomial Hypersurface over any field of arbitrary positive characteristic is rational. As an example, I also prove the well known fact that for 1-dimensional Binomial Hypersurfaces, the Hilbert-Kunz multiplicity is a positive integer and give a precise account of the integer.
In this paper, we work with certain families of ideals called $p$-families in rings of prime characteristic. This family of ideals is present in the theories of tight closure, Hilbert-Kunz multiplicity, and $F$-signature. For each $p$-family of ideals, we attach a Euclidean object called $p$-body, which is analogous to the Newton Okounkov body associated with a graded family of ideals. Using the combinatorial properties of $p$-bodies and algebraic properties of the Hilbert-Kunz multiplicity, we establish in this paper a Volume = Multiplicity formula for $p$-families of $\mathfrak{m}_{R}$-prima…
We present a unified approach to the study of Hilbert-Kunz multiplicity, F-signature, and related limits governed by Frobenius and Cartier linear actions in positive characteristic commutative algebra. We introduce general techniques that give vastly simplified proofs of existence, semicontinuity, and positivity. Furthermore, we give an affirmative answer to a question of Watanabe and Yoshida allowing the F-signature to be viewed as the infimum of relative differences in the Hilbert-Kunz multiplicites of the cofinite ideals in a local ring.
This paper establishes uniform bounds in characteristic $p$ rings which are either F-finite or essentially of finite type over an excellent local ring. These uniform bounds are then used to show that the Hilbert-Kunz length functions and the normalized Frobenius splitting numbers defined on the Spectrum of a ring converge uniformly to their limits, namely the Hilbert-Kunz multiplicity function and the F-signature function. From this we establish that the F-signature function is lower semi-continuous. We also give a new proof of the upper semi-continuity of Hilbert-Kunz multiplicity, which was …
In preparing the paper "Some extensions of Hilbert-Kunz multiplicity", we had occasion to perform an intricate set of computations pertaining to a single illustrative example. In the end, we have decided not to include the computations in the main paper, but instead to publish them here for later reference. In particular, the current preprint gives an example where the converse to one of our main theorems holds, even though it does not fit into any of the previously known cases where a converse holds.
Given a commutative local ring $(R,\mathfrak m)$ and an ideal $I$ of $R$, a family of quotients of the Rees algebra $R[It]$ has been recently studied as a unified approach to the Nagata's idealization and the amalgamated duplication and as a way to construct interesting examples, especially integral domains. When $R$ is noetherian of prime characteristic, we compute the Hilbert-Kunz function of the members of this family and, provided that either $I$ is $\mathfrak{m}$-primary or $R$ is regular and F-finite, we also find their Hilbert-Kunz multiplicity. Some consequences and examples are explor…
Semistability and Hilbert-Kunz multiplicities for curves
研究了正特征非奇异曲线的Hilbert-Kunz重数。我们分析了曲线及其丰富线丛的核层的Frobenius半稳定性与HK重数之间的关系。这将导致一个下界，实现当且仅当核心层是Frobenius半稳定的，否则公式的HK多重性的参数测量失败的Frobenius半稳定。作为副产品，给出曲线上的向量丛的显式示例，其$n$次迭代Frobenius拉回不是半稳定的，而其$(n-1)$次这样的拉回是半稳定的，其中$n>0$是任意的。
Perfectoid signature, perfectoid Hilbert-Kunz multiplicity, and an application to local fundamental groups
本文利用完美oid化函子定义了混合特征下的完美oid签名与Hilbert-Kunz重数，证明其与等特征经典理论一致，并用于刻画正则性、BCM闭包及BCM-正则性，进而证明BCM-正则环具有有限局部平展基本群。
We provide suitable conditions under which the asymptotic limit of the Hilbert-Samuel coefficients of the Frobenius powers of an $\mathfrak{m}$-primary ideal exists in a Noetherian local ring $(R,\mathfrak{m})$ with prime characteristic $p>0.$ This, in turn, gives an expression of the Hilbert-Kunz multiplicity of powers of the ideal. We also prove that for a face ring $R$ of a simplicial complex and an ideal $J$ generated by pure powers of the variables, the generalized Hilbert-Kunz function $\ell(R/(J^{[q]})^k)$ is a polynomial for all $q,k$ and also give an expression of the generalized Hilb…
Derivation module and the Hilbert-Kunz multiplicity of the co-ordinate ring of a projective monomial curve
针对参数对应等差数列的射影单项式曲线坐标环，本文显式构造其导子模的极小生成元，并给出 Hilbert–Kunz 重数公式。
In a formally unmixed Noetherian local ring, if the colength and multiplicity of an integrally closed ideal agree, then $R$ is regular. We deduce this using the relationship between multiplicity and various ideal closure operations.
The Hilbert-Kunz function in graded dimension two
设R表示特征为p的有限域的代数闭包K上的二维正规标准分次整环，I表示齐次准素理想.证明了I的Hilbert-Kunz函数的形式为= e_{HK}（I）q^{2} + g（q），其有理Hilbert-Kunz重数为e_{HK}（I），g（q）是最终的周期函数.
The paper investigates the behavior of Hilbert-Samuel and Hilbert-Kunz multiplicities in families of ideals. It is shown that Hilbert-Samuel multiplicity is upper semicontinuous almost generally and that Hilbert-Kunz multiplicity is upper semicontinuous in families of finite type. Surprisingly, our machinery can be applied for families over Z and yields a partial solution to the question about characteristic zero Hilbert-Kunz multiplicity posed by Brenner, Li, and Miller. Another application is that for an affine ring the infimum in the definition of F-rational signature, an invariant defined …
Let $(R,\mathfrak m)$ be a local ring of characteristic $p>0$ and $M$ a finitely generated $R$-module. In this note we consider the limit: $\lim_{n\to \infty} \frac{\ell(H^0_{\mathfrak m}(F^n(M)))}{p^{n\dim R}} $ where $F(-)$ is the Peskine-Szpiro functor. A consequence of our main results shows that the limit always exists when $R$ is excellent and has isolated singularity. Furthermore, if $R$ is a complete intersection, then the limit is 0 if and only if the projective dimension of $M$ is less than the Krull dimension of $R$. We exploit this fact to give a quick proof that if $R$ is a comple…
Hilbert-Kunz multiplicity and $F$-signature can disagree
本文通过计算素特征域上直纹面丰富锥的 F-签名函数，构造出强 F-正则环，证明 Hilbert-Kunz 重数与 F-签名可在同一环的不同极大理想处对奇点给出相反排序，并纠正了 Hirzebruch 曲面 F-签名计算的既有错误。
For a toric pair $(X, D)$, where $X$ is a projective toric variety of dimension $d-1\geq 1$ and $D$ is a very ample $T$-Cartier divisor, we show that the Hilbert-Kunz density function $HKd(X, D)(λ)$ is the $d-1$ dimensional volume of ${\overline {\mathcal P}}_D \cap \{z= λ\}$, where ${\overline {\mathcal P}}_D\subset {\mathbb R}^d$ is a compact $d$-dimensional set (which is a finite union of convex polytopes). We also show that, for $k\geq 1$, the function $HKd(X, kD)$ can be replaced by another compactly supported continuous function $φ_{kD}$ which is `linear in $k$'. This gives the formula f…
A recent continuous family of multiplicity functions on local rings was introduced by Taylor interpolating between Hilbert-Samuel and Hilbert-Kunz multiplicities. The obvious goal is to use this as a tool for deforming results from one to the other. The values in this family which do not match these classic variants however are not known yet to be well-behaved. This article explores lower bounds for these intermediate multiplicities as well as gives evidence for analogies of the Watanabe-Yoshida minimality conjectures for unmixed singular rings.
The Hilbert-Kunz density functions of quadric hypersurfaces
本文研究了二次超曲面的 Hilbert-Kunz 密度函数，证明其为分段多项式，并借此证实了 Watanabe-Yoshida 和 Yoshida 关于 Hilbert-Kunz 重数下界与特征递减性的猜想。
Transcendental Hilbert-Kunz Multiplicities
该研究证明普通希尔伯特-孔兹重数可呈超越性，在特征p>2的不可数代数闭域上构造出满足对应超越性条件的正规分次整环及相关理想。
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