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The problem of giving a (CR-)geometric description of the best possible order of a subelliptic estimate at a boundary point in the $\bar\partial$-Neumann problem is largely open. In this paper, we introduce a novel technique based on a "$\bar\partial$-uncertainty principle" and, as an application, we determine the sharp order of subellipticity at the origin for a large class of Kohn's special domains in ambient dimension $\leq 5$., Comment: 37 pages

We employ tools from complex analysis to construct the $*$-logarithm of a quaternionic slice regular function. Our approach enables us to achieve three main objectives: we compute the monodromy associated with the $*$-exponential; we establish sufficient conditions for the $*$-product of two $*$-exponentials to also be a $*$-exponential; we calculate the slice derivative of the $*$-exponential of a regular function., Comment: 27 pages

We investigate a few aspects of the notion of Levi core, introduced by the authors in a previous work: a basic finiteness question, the connections with Kohn's algorithm and with Catlin's property (P).

We show how the formalism of Levi currents on complex manifolds, as introduced by Sibony, can be used to study the analytic structure of singular sets associated to families of plurisubharmonic functions, in the sense of Slodkowski.

The aim of this paper is to prove that a large class of quaternionic slice regular functions result to be (ramified) covering maps. By means of the topological implications of this fact and by providing further topological structures, we are able to give suitable natural conditions for the existence of $k$-th $\star$-roots of a slice regular function. Moreover, we are also able to compute all the solutions which, quite surprisingly, in the most general case, are in number of $k^2$. The last part is devoted to compute the monodromy and to present a technique to compute all the $k^2$ roots starting from one of them., Comment: New version: published in Journal of Geometric Analysis; 37 pages

We introduce a new geometrical invariant of CR manifolds of hypersurface type, which we dub the "Levi core" of the manifold. When the manifold is the boundary of a smooth bounded pseudoconvex domain, we show how the Levi core is related to two other important global invariants in several complex variables: the Diederich--Forn{\ae}ss index and the D'Angelo class (namely the set of D'Angelo forms of the boundary). We also show that the Levi core is trivial whenever the domain is of finite-type in the sense of D'Angelo, or the set of weakly pseudoconvex points is contained in a totally real submanifold, while it is nontrivial if the boundary contains a local maximum set. As corollaries to the theory developed here, we prove that for any smooth bounded pseudoconvex domain with trivial Levi core the Diederich--Forn{\ae}ss index is one and the $\overline{\partial}$-Neumann problem is exactly regular (via a result of Kohn and its generalization by Harrington). Our work builds on and expands recent results of Liu and Adachi--Yum., Comment: 40 pages

We provide a family of isolated tangent to the identity germs $f:(\mathbb{C}^3,0) \to (\mathbb{C}^3,0)$ which possess only degenerate characteristic directions, and for which the lift of $f$ to any modification (with suitable properties) has only degenerate characteristic directions. This is in sharp contrast with the situation in dimension $2$, where any isolated tangent to the identity germ $f$ admits a modification where the lift of $f$ has a non-degenerate characteristic direction. We compare this situation with the resolution of singularities of the infinitesimal generator of $f$, showing that this phenomenon is not related to the non-existence of complex separatrices for vector fields of Gomez-Mont and Luengo. Finally, we describe the set of formal $f$-invariant curves, and the associated parabolic manifolds, using the techniques recently developed by L\'opez-Hernanz, Raissy, Rib\'on, Sanz S\'anchez, Vivas., Comment: 42 pages, 3 figures

A complex manifold $X$ is \emph{weakly complete} if it admits a continuous plurisubharmonic exhaustion function $\phi$. The minimal kernels $\Sigma_X^k, k \in [0,\infty]$ (the loci where are all $\mathcal{C}^k$ plurisubharmonic exhaustion functions fail to be strictly plurisubharmonic),introduced by Slodkowski-Tomassini, and the Levi currents, introduced by Sibony, are both concepts aimed at measuring how far $X$ is from being Stein. We compare these notions, prove that all Levi currents are supported by all the $\Sigma_X^k$'s, and give sufficient conditions for points in $\Sigma_X^k$ to be in the support of some Levi current. When $X$ is a surface and $\phi$ can be chosen analytic, building on previous work by the second author, Slodkowski, and Tomassini,we prove the existence of a Levi current precisely supported on $\Sigma_X^\infty$, and give a classification of Levi currents on $X$. In particular,unless $X$ is a modification of a Stein space, every point in $X$ is in the support of some Levi current.

In this paper, we study the (complex) geometry of the set $S$ of the square roots of $-1$ in a real associative algebra $A$, showing that $S$ carries a natural complex structure, given by an embedding into the Grassmannian of $\mathbb{C}\otimes A$. With this complex structure, slice-regular functions on $A$ can be lifted to holomorphic maps from $\mathbb{C}\times S$ to $\mathbb{C}\otimes A \times S$ and the values of the original slice-regular functions are recovered by looking at how the image of such holomorphic map intersects the leaves of a particular foliation on $\mathbb{C}\otimes A \times S$, constructed in terms of incidence varieties. In this setting, the quadratic cone defined by Ghiloni and Perotti is obtained by considering some particular (compact) subvarieties of $S$, defined in terms of some inner product on $A$. Moreover, by defining an analogue of the stereographic projection, we extend the construction of the twistor transform, introduced by Gentili, Salamon and Stoppato, to the case of an associative algebra, under the hypothesis of the existence of sections for a given projective bundle. Finally, we introduce some more general classes of "slice-regular" functions to which the present theory applies in all qualitative aspects.

In this paper we study mapping properties of Toeplitz-like operators on weighted Bergman spaces of bounded strongly pseudconvex domains in $\mathbb{C}^n$. In particular we prove that a Toeplitz operator built using as kernel a weighted Bergman kernel of weight $\beta$ and integrating against a measure $\mu$ maps continuously (when $\beta$ is large enough) a weighted Bergman space $A^{p_1}_{\alpha_1}(D)$ into a weighted Bergman space $A^{p_2}_{\alpha_2}(D)$ if and only if $\mu$ is a $(\lambda,\gamma)$-skew Carleson measure, where $\lambda=1+\frac{1}{p_1}-\frac{1}{p_2}$ and $\gamma=\frac{1}{\lambda}\left(\beta+\frac{\alpha_1}{p_1}-\frac{\alpha_2}{p_2}\right)$. This theorem generalizes results obtained by Pau and Zhao on the unit ball, and extends and makes more precise results obtained by Abate, Raissy and Saracco on a smaller class of Toeplitz operators on bounded strongly pseudoconvex domains., Comment: 21 pages. arXiv admin note: text overlap with arXiv:1710.01631

In this paper, we want to study the link between the presence of compact objects with some analytic structure and the global geometry of a weakly complete surface. We begin with a brief survey of some now classic results on the local geometry around a (complex) curve, which depends on the sign of its self-intersection and, in the flat case, on some more refined invariants (see the works of Grauert, Suzuki, Ueda). Then, we recall some results about the propagation of compact curves and the existence of holomorphic functions (from the works of Nishino and Ohsawa). With such considerations in mind, we give an overview of the classification results for weakly complete surfaces that we obtained in two joint papers with Slodkowski (see [MST18], [MST17] and we present some new results which stem from this somehow more local (or less global) viewpoint (see Sections 3.2, 3.3 and 4).

In previous works, G. Tomassini and the authors studied and classified complex surfaces admitting a real-analytic pluri-subharmonic exhaustion function; let $X$ be such a surface and $D\subseteq X$ a domain admitting a \emph{continuous} plurisubharmonic exhaustion function: what can be said about the geometry of $D$? If the exhaustion of $D$ is assumed to be smooth, the second author already answered this question; however, the continuous case is more difficult and requires different methods. In the present paper, we address such question by studying the local maximum sets contained in $D$ and their interplay with the complex geometric structure of $X$; we conclude that, if $D$ is not a modification of a Stein space, then it shares the same geometric features of $X$.

In this short note, we collect some results regarding the Remmert reduction of holomorphically convex space and its application to a variation of the usual union problem. Classically, the union problem asks the following question: is a complex space, which is an increasing union of Stein subspaces $X_1\Subset X_2\Subset\cdots$, a Stein space itself? The variation we are interested in is the following: is a complex space, which is an increasing union of holomorphically convex subspaces $X_1\Subset X_2\Subset\cdots$, holomorphically convex itself? The results presented here are close analogues of (some of) those alredy present in the literature for the Stein case; our aim is only to collect such material for reference, as we consider it well known.

Slice-regular functions of a quaternionic variable have been studied extensively in the last 12 years, resulting, in many ways, quite close to classical holomorphic functions of a complex variable; indeed, there is a correspondence between slice-regular functions and a certain family of holomorphic maps from the complex plane to $\mathbb{C}^4$, as noted by Ghiloni and Perotti. However, such a construction does not seem to offer any insight on the behaviour of slice-regular functions, due to the lack of a connection between the values of the holomorphic map and the values of the associated sliceregular function. The aim of this work is to show that there is indeed a (complex) geometric way to relate the values of this two functions, thus relating more deeply the world of holomorphic functions with that of slice-regular functions.

The aim of this short note is to investigate the geometry of weakly complete subdomains of Grauert type surfaces, i.e. open connected sets D, sitting inside a Grauert type surface X, which admit a smooth plurisubharmonic exhaustion function. We prove that they are either modifications of Stein spaces or Grauert type surfaces themselves and we apply these results to the special case of Hopf surfaces.

The name of Oka principle, or Oka-Grauert principle, is traditionally used to refer to the holomorphic incarnation of the homotopy principle: on a Stein space, every problem that can be solved in the continuous category, can be solved in the holomorphic category as well. In this note, we begin the study of the same kind of questions on a Levi-flat manifold; more precisely, we try to obtain a classification of CR-bundles on a semiholomorphic foliation of type (n, 1). Our investigation should only be considered a preliminary exploration, as it deals only with some particular cases, either in terms of regularity or bidegree of the bundle, and partial results.

In this paper we study the additive splitting associated to the quaternionic Cauchy transform defined by the Cauchy formula of slice hyperholomorphic functions. Moreover, we introduce and study the analogue of the fundamental solution of the global operator of slice hyperholomorphic functions. We state our results in the quaternionic setting but several results hold for Clifford algebra-valued function with minor changes in the proofs.

In a previous work, we classified weakly complete surfaces which admit a real analytic plurisubharmonic exhaustion function; we showed that, if they are not proper over a Stein space, then they admit a pluriharmonic function, with compact Levi-flat level sets foliated with dense complex leaves. We called these Grauert type surfaces. In this note we investigate some properties of these surfaces. Namely, we prove that the only compact curves that can be contained in them are negative in the sense of Grauert and that the level sets of the pluriharmonic function are connected, thus completing the analogy with the Remmert-Stein reduction of a holomorphically convex space. Moreover, in our classification Theorem, we had to pass to a double cover to produce the pluriharmonic function; the last part of the present paper is devoted to the construction of an example where it is necessary to do so.

A weakly complete space is a complex space admitting a (smooth) plurisubharmonic exhaustion function. In this paper, we classify those weakly complete complex surfaces for which such exhaustion function can be chosen real analytic: they can be modifications of Stein spaces or proper over a non compact (possibly singular) complex curve or foliated with real analytic Levi-flat hypersurfaces which in turn are foliated by dense complex leaves (these we call surfaces of Grauert type). In the last case, we also show that such Levi-flat hypersurfaces are in fact level sets of a global proper pluriharmonic function, up to passing to a holomorphic double cover of the space. An example of Brunella shows that not every weakly complete surface can be endowed with a real analytic plurisubharmonic exhaustion function. Our method of proof is based on the careful analysis of the level sets of the given exhaustion function and their intersections with the minimal singular set, i.e the set where every plurisubharmonic exhaustion function has a degenerate Levi form.

A semiholomorphic foliations of type (n, d) is a differentiable real manifold X of dimension 2n + d, foliated by complex leaves of complex dimension n. In the present work, we introduce an appropriate notion of pseudoconvexity (and consequently, q-completeness) for such spaces, given by the interplay of the usual pseudoconvexity, along the leaves, and the positivity of the transversal bundle. For 1-complete real analytic semiholomorphic foliations, we obtain a vanishing theorem for the CR cohomology, which we use to show an extension result for CR functions on Levi flat hypersurfaces and an embedding theorem in C^N . In the compact case, we introduce a notion of weak positivity for the transversal bundle, which allows us to construct a real analytic embedding in CP^N ., Comment: 27 pages

In this paper we consider a perturbation of the Ricci solitons equation proposed in \cite{jpb1} and studied in \cite{CaMa} and we classify noncompact gradient shrinkers with bounded nonnegative sectional curvature., Comment: Minor corrections

Metric currents are, in a certain sense, a metric analogous of flat currents, therefore are related to the geometry of the space and of their support. In this short note, we try to give some evidence for the previous statement, by showing that the homology which can be defined by means of metric normal currents coincides, on nice enough metric spaces, with the usual singular homology., Comment: 7 pages

We establish a "low rank property" for Sobolev mappings that pointwise solve a first order nonlinear system of PDEs, whose smooth solutions have the so-called "contact property". As a consequence, Sobolev mappings from an open set of the plane, taking values in the first Heisenberg group and that have almost everywhere maximal rank must have images with positive 3-dimensional Hausdorff measure with respect to the sub-Riemannian distance of the Heisenberg group. This provides a complete solution to a question raised in a paper by Z. M. Balogh, R. Hoefer-Isenegger and J. T. Tyson. Our approach differs from the previous ones. Its technical aspect consists in performing an "exterior differentiation by blow-up", where the standard distributional exterior differentiation is not possible. This method extends to higher dimensional Sobolev mappings taking values in higher dimensional Heisenberg groups., Comment: 12 pages

In 2000, Ambrosio and Kirchheim, with the paper "Currents in metric spaces", settled the foundations of a theory of currents on metric spaces and used it to pose and solve Plateau's problem in a wide class of Banach spaces. Following an idea of De Giorgi, they gave a new definition of current which was meaningful on any metric space. A metric current is a multilinear functional on (k+1)-tuples of Lipschitz functions (with the first one bounded) satisfying a continuity property, a locality property and a finite mass property. The second chapter of the thesis is devoted to the theory of local metric currents. We introduce the basic concepts and adapt the theory to the complex case, defining the bidimension, the Dolbeault decomposition and related notions. A characterization of holomorphic currents is given. The development of a Sobolev theory on singular space is our main concern in the third chapter. We give a characterization of Sobolev functions in terms of their behavior and growth on the regular part; this leads to a capacity theory for the singular set which allows us to obtain an approximation result, using functions with support in the regular part. The second part of the chapter deals with the L2 theory on singular spaces. In the fourth chapter, some applications of the theory are discussed. We solve Cauchy-Riemann equation on completely reducible singularities, by means of a structure theorem for metric currents; we also treat the equation in Lp on complex curves and outline a possible approach for the same problem on complex spaces which can be embedded as divisors in C^n. The final chapter tries to spread some light on the complex geometry in infinite dimensional spaces. After solving the Cauchy-Riemann equation in Banach spaces, in terms of the quasi-local metric currents, we turn to the study of positive currents, holomorphic chains and their boundaries in Hilbert spaces., Comment: PhD Thesis

We treat the boundary problem for complex varieties with isolated singularities, of complex dimension greater than or equal to 3, non necessarily compact, which are contained in strongly convex, open subsets of a complex Hilbert space H. We deal with the problem by cutting with a family of complex hyperplanes in the fashion of [2] and applying the first named author's result for the compact case [13]., Comment: 13 pages. arXiv admin note: substantial text overlap with arXiv:math/0503430

We present some results concerning currents of integration on finite-dimensional analytic spaces in Hilbert spaces, using the setting of metric currents. In particular, we obtain the characterization of such currents as positive closed $(k,k)-$rectifiable currents and solve the boundary problem for holomorphic chains.

We compute the evolution equation of the Cotton and the Bach tensor under the Ricci flow of a Riemannian manifold, with particular attention to the three dimensional case, and we discuss some applications., Comment: 28 pages

The aim of this paper is to show two applications of metric currents to complex analysis. After recalling the basic definitions, we give a detailed proof of the comparison theorem between metric currents and classical ones on a manifold. In Section 3 we introduce the concept of bidimension for a metric current on a finite dimensional space, showing that the usual properties of $(p,q)-$currents still hold, except for the existence of a Dolbeault decomposition. Section 4 is devoted to the analysis of a particular class of complex spaces, whose structure allows us to give a structure theorem for currents, solve the Cauchy-Riemann equation and characterize holomorphic currents. In Section 5, we introduce the concept of bidimension of (global) metric currents on a Banach space and relate it to the behaviour of the finite dimensional projections of the currents. In section 6 we define a new class of currents, the \emph{quasi-local} metric currents, which are usual metric currents when restricted to bounded sets, and we give a definition of $(p,q)-$current in this new class. The last Section shows how to employ these newly defined quasi-local currents in order to obtain a solution to the equation $\debar U=T$, when $T$ is of bidimension $(0,q)$ and its support is bounded; finally, we extend the result to a current $T$ with generic bidimension, $\debar-$closed, with bounded support.

In one complex variable, the existence of a compactly supported solution to the Cauchy-Riemann equation is related to the vanishing of certain integrals of the data; trying to generalize this approach, we find an explicit construction, via convolution, for a compactly supported solution in $\mathbb{C}^n$, which allows to estimate the $L^p$ norm of the solution. We also investigate the possible generalizations of this method to domains of the form $P\setminus Z$, where $P$ is a polydisc and $Z$ is the zero locus of some holomorphic function.