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Let $M$ and $N$ be modules over a commutative ring $R$ with $N$ Noetherian. We define the injective capacity of $M$ with respect to $N$ over $R$ to be the supremum of the values $t$ for which $N^{\oplus t}$ embeds into $M$. In a dual fashion, we deem the number of cogenerators of $N$ with respect to $M$ over $R$ to be the infimum of the numbers $t$ for which $N$ embeds into $M^{\oplus t}$. We demonstrate that the global injective capacity is the infimum of its local analogues and that the global number of cogenerators is the supremum of the corresponding local invariants. We also prove enhanced versions of these statements and consider the graded case., Comment: 9 pages

We extend the notion of Frobenius Betti numbers and F-splitting ratio to large classes of finitely generated modules over rings of prime characteristic, which are not assumed to be local. We also prove that the strong F-regularity of a pair $(R,\mathscr{D})$, where $\mathscr{D}$ is a Cartier algebra, is equivalent to the positivity of the global F-signature ${\rm s}(R,\mathscr{D})$ of the pair. This extends a result previously proved by these authors, by removing an extra assumption on the Cartier algebra., 31 pages

Let $(R,\mathfrak{m})$ be a Noetherian local ring, and let $M$ be a finitely generated $R$-module of dimension $d$. We prove that the set $\left\{\frac{l(M/IM)}{e(I, M)} \right\}_{\sqrt{I}=\mathfrak{m}}$ is bounded below by ${1}/{d!e(\overline{R})}$ where $\overline{R}=R/Ann(M)$. Moreover, when $\widehat{M}$ is equidimensional, this set is bounded above by a finite constant depending only on $M$. The lower bound extends a classical inequality of Lech, and the upper bound answers a question of St��ckrad--Vogel in the affirmative. As an application, we obtain results on uniform behavior of the lengths of Koszul homology modules., 24 pages, minor changes. To appear in Advances in Mathematics

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., 7 pages, comments welcome. arXiv admin note: substantial text overlap with arXiv:1103.4730

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 give truly different numerical results) and explore special cases where equivalence with matching tight closure can be shown. All of our measures derive ultimately from Hilbert-Kunz multiplicity., 22 pages

This paper studies Frobenius powers of parameter ideals in a commutative Noetherian local ring $R$ of prime characteristic $p$. For a given ideal $\fa$ of $R$, there is a power $Q$ of $p$, depending on $\fa$, such that the $Q$-th Frobenius power of the Frobenius closure of $\fa$ is equal to the $Q$-th Frobenius power of $\fa$. The paper addresses the question as to whether there exists a {\em uniform} $Q_0$ which `works' in this context for all parameter ideals of $R$ simultaneously. In a recent paper, Katzman and Sharp proved that there does exists such a uniform $Q_0$ when $R$ is Cohen--Macaulay. The purpose of this paper is to show that such a uniform $Q_0$ exists when $R$ is a generalized Cohen--Macaulay local ring. A variety of concepts and techniques from commutative algebra are used, including unconditioned strong $d$-sequences, cohomological annihilators, modules of generalized fractions, and the Hartshorne--Speiser--Lyubeznik Theorem employed by Katzman and Sharp in the Cohen--Macaulay case., This is to appear in the Journal of Algebra