# Complex Analysis and Complex Geometry (09w5033)

Arriving in Banff, Alberta Sunday, May 31 and departing Friday June 5, 2009

## Organizers

Dan Coman (Syracuse University)

Finnur Larusson (University of Adelaide)

Stefan Nemirovski (Steklov Institute)

Rasul Shafikov (University of Western Ontario)

## Objectives

The objective of the workshop is to bring together an international group of experts in complex analysis and complex geometry to report on and discuss recent progress and open problems in the area and thus foster interaction and collaboration between researchers in diverse subfields. A number of current and recent developments, described in some detail below, make such a workshop useful and timely.

We view complex analysis on the one hand and complex geometry on the other as two aspects of the same subject. The two are inseparable, as most work in the area involves interplay between analysis and geometry. The fundamental objects of the theory are complex manifolds and, more generally, complex spaces, holomorphic functions on them, and holomorphic maps between them. Holomorphic functions can be defined in three equivalent ways as complex-differentiable functions, as sums of complex power series, and as solutions of the homogeneous Cauchy-Riemann equation. The threefold nature of differentiability over the complex numbers gives complex analysis its distinctive character and is the ultimate reason why it is linked to so many areas of mathematics.

Plurisubharmonic functions are not as well known to nonexperts as holomorphic functions. They were first explicitly defined in the 1940s, but they had already appeared in attempts to geometrically describe domains of holomorphy at the very beginning of several complex variables in the first decade of the 20th century. Since the 1960s, one of their most important roles has been as weights in a priori estimates for solving the Cauchy-Riemann equation. They are intimately related to the complex Monge-Amp`ere equation, the second partial differential equation of complex analysis. There is also a potential-theoretic aspect to plurisubharmonic functions, which is the subject of pluripotential theory.

In the early decades of the modern era of the subject, from the 1940s into the 1970s, the notion of a complex space took shape and the geometry of analytic varieties and holomorphic maps was developed. Also, three approaches to solving the Cauchy-Riemann equations were discovered and applied. First came a sheaf-theoretic approach in the 1950s, making heavy use of homological algebra. Hilbert space methods appeared in the early 1960s and integral formulas around 1970 through interaction with partial differential equations and harmonic analysis. The complex Monge-Amp`ere equation came to the fore in the late 1970s with Yau's solution of the Calabi conjectures and Bedford and Taylor's work on the Dirichlet problem.

Most current work in complex analysis and complex geometry can be seen as being focused on one or both of the two fundamental partial differential equations, Cauchy-Riemann and Monge-Amp`ere, in the setting of Euclidean space or more general complex manifolds. The past ten years have seen an increasing thrust towards extending both the theory and its applications to singular spaces, to almost complex manifolds, and to infinite-dimensional manifolds. These generalizations will be a major focus of the workshop.

Today, as before, complex analysis and complex geometry is a highly interdisciplinary field. The foundational work described above has been followed by a broad range of research at the interfaces with a number of other areas, such as algebraic geometry, functional analysis, partial differential equations, and symplectic geometry, to name a few. Complex analysts and complex geometers share a common toolkit, but find inspiration and open problems in many areas of mathematics. It would be beneficial to bring together a group of experts from diverse subfields to discuss recent results and work in progress and to share ideas on open questions. A single workshop cannot do justice to the breadth and depth of contemporary complex analysis and complex geometry. The organizers have chosen a coherent collection of interrelated topics for the workshop, representing, in their view, some of the most vibrant developments in the subject today. Each of the five topics will be briefly introduced in general terms, followed by a few recent highlights.

{bf 1. Analytic methods in complex algebraic geometry} are based on increasingly sophisticated ways of solving the Cauchy-Riemann equation (often also called the $barpartial$-equation) with $L^2$-estimates using plurisubharmonic weights in geometric settings. Yum-Tong Siu has long been a leader in this area. His announcement of an analytic proof of the finite generation of the canonical ring of a smooth complex projective variety of general type in October 2006 ({tt arXiv:math/0610740}) came on the heels of an algebraic proof by Birkar, Cascini, Hacon, and McKernan. This is a milestone in algebraic geometry.

Bo Berndtsson and Mihai Paun use analytic methods to obtain a nearly optimal criterion for the pseudoeffectivity of relative canonical bundles and give several applications in algebraic geometry ({tt arXiv:math/0703344}). Shigeharu Takayama uses analytic techniques, including multiplier ideal sheaves, to extend Siu's celebrated result on the invariance of plurigenera from the smooth case to the case of fibres with canonical singularities ({it J. Algebraic Geom.} {bf 16} (2007) 1--18). So far, there is no known algebraic proof of the full result.

Currents are differential forms with distribution coefficients; closed currents satisfying a certain positivity condition are objects of fundamental importance that generalize both K"ahler forms and analytic subvarieties. New work of Tien-Cuong Dinh and Nessim Sibony advances the basic theory of currents (intersections, pullbacks, etc.) and has many potential applications ({tt arXiv:math/0606248}, {tt arXiv:math/0703702}).

{bf 2. Pluripotential theory and the Monge-Amp`ere equation.} Pluripotential theory on compact K"ahler manifolds, based on the notion of a quasiplurisubharmonic function, has been developed by Vincent Guedj and Ahmed Zeriahi since 2004 (starting with {tt arXiv:math/0401302}). Whereas plurisubharmonic functions have a positive Levi form by definition and are constant on compact manifolds, quasiplurisubharmonic functions are allowed to have a negative Levi form down to a fixed lower bound and lead to a fruitful pluripotential theory in a compact setting. Guedj, Zeriahi, and Philippe Eyssidieux posted a major application of this theory in March 2006 ({tt arXiv:math/0603431}). They extended the work of Aubin and of Yau on the complex Monge-Amp`ere equation to certain singular settings and proved that the canonical model of a smooth complex projective variety of general type (proved to exist soon afterwards by Birkar et al. and by Siu --- this is equivalent to finite generation of the canonical ring) has a K"ahler-Einstein metric of negative Ricci curvature. This is only one example, albeit a very important one, of current work on the complex Monge-Amp`ere equation.

The highly nonlinear nature of the Monge-Amp`ere operator presents many challenges. Proper understanding of its maximal domain of definition in the local case of a domain in Euclidean space was obtained only recently in work of Zbigniew Bl ocki ({it Amer. J. Math.} {bf 128} (2006) 519--530). Surprisingly, very recent work of Guedj, Zeriahi, and Dan Coman has shown the domain of definition to be much larger in the global case of a compact K"ahler manifold ({tt arXiv:0705.4529}).

Coman and Evgeny Poletsky have derived new Bernstein, Bezout, and Markov inequalities using pluripotential theory and applied them to transcendental number theory ({it Invent. Math.} {bf 170} (2007) 103--145, {tt arXiv:math/0403420}). Their results have been generalized by Alexander Brudnyi ({tt arXiv:0708.3449}). In a series of papers, the earliest posted in 2002, Charles Favre and Mattias Jonsson have made a deep study of the singularities of plurisubharmonic functions and multiplier ideals in two dimensions using the novel concept of a tree of valuations (see Springer Lecture Notes in Mathematics, vol. 1853, 2004). In a new paper with Sebastien Boucksom, they have extended some of their work to higher dimensions ({tt arXiv:math/0702487}). A connection with probability theory appears in recent work of Thomas Bloom and Bernard Shiffman ({tt arXiv:math/0605739)} and of Robert Berman ({tt arXiv:math/0608226)}, who use various techniques of pluripotential theory to study zeros of random polynomials and, more generally, random sections of holomorphic line bundles.

{bf 3. The Cauchy-Riemann equation on singular spaces.} Existing methods for solving the Cauchy-Riemann equation are largely restricted to smooth spaces. Consequently, the central problem of classical several complex variables, the Levi problem, which asks whether Steinness is a local property and was solved for manifolds decades ago, is still open for singular spaces. Progress in this area has been slow and difficult. Recently, John Erik Fornae ss, Nils O vrelid, and Sophia Vassiliadou have been able to solve the Cauchy-Riemann equations with $L^2$-estimates in certain singular settings ({tt arXiv:math/0401171}, {tt arXiv:math/0610569}).

{bf 4. Almost complex geometry.} In a seminal paper of 1985, Gromov introduced almost complex structures and pseudoholomorphic curves into symplectic topology. Interaction between complex geometry and symplectic geometry began in earnest with the work of Sergey Ivashkovich and Vsevolod Shevchishin in the late 1990s ({tt arXiv:math/9912046}). There is now a growing body of work concerned with extending concepts and results from complex analysis and complex geometry to the almost complex case. Often the non-integrable case requires new methods that shed light on the integrable case. Notable new work includes a paper by Bernard Coupet, Alexander Tumanov, and Alexander Sukhov on proper pseudoholomorphic discs ({tt arXiv:0704.0124}), a long paper on fundamentals of local almost complex geometry by Coupet, Sukhov, and Herv'e Gaussier ({tt arXiv:math/0701576}), a paper by Xianghong Gong and Jean-Pierre Rosay on removable singularities of pseudoholomorphic maps ({tt arXiv:0708.1726}), and a paper by Ivashkovich and Shevchishin on almost complex structures that are merely Lipschitz ({tt arXiv:0707.0771})

{bf 5. Infinite-dimensional complex geometry.} Complex analysis in infinite dimensions languished outside the mainstream until L'aszl'o Lempert commenced a major research program in the mid-1990s. Generalized loop spaces (spaces of smooth maps from a compact smooth manifold into a finite-dimensional complex manifold) are examples of infinite-dimensional complex manifolds; their importance in physics provides strong motivation for Lempert's program. Fundamental notions, including Dolbeault cohomology, coherence of analytic sheaves, and holomorphic approximation, have been brought into an infinite-dimensional setting by Lempert, in part with coauthors Endre Szab'o and Ning Zhang ({tt arXiv:math/0403405}, {tt arXiv:math/0507449}, {tt arXiv:math/0507549}). Imre Patyi has studied the Oka principle for infinite-dimensional complex manifolds ({tt arXiv:math/0509557}).

{bf Timeliness and community interest.} Why is this a timely proposal for 2009? Most of the work we have cited is brand new, posted in 2006 or 2007, and not yet in print. We want to hold an international workshop on these developments as soon as possible, and the earliest it can be, at least if we want to avail ourselves of the excellent facilities and inspiring natural setting of BIRS, is 2009.

There is strong community interest in the workshop proposed here. Nine days before submitting this proposal, we sent out an e-mail to a group of people, including most of the authors cited above, and asked: {it If the workshop goes ahead and is scheduled at a time suitable for you, would you be interested in attending it?} Within a week, 37 people had replied and expressed an interest in the workshop. Thus, the 41 people on our list of possible participants have all confirmed their interest in the workshop.

We view complex analysis on the one hand and complex geometry on the other as two aspects of the same subject. The two are inseparable, as most work in the area involves interplay between analysis and geometry. The fundamental objects of the theory are complex manifolds and, more generally, complex spaces, holomorphic functions on them, and holomorphic maps between them. Holomorphic functions can be defined in three equivalent ways as complex-differentiable functions, as sums of complex power series, and as solutions of the homogeneous Cauchy-Riemann equation. The threefold nature of differentiability over the complex numbers gives complex analysis its distinctive character and is the ultimate reason why it is linked to so many areas of mathematics.

Plurisubharmonic functions are not as well known to nonexperts as holomorphic functions. They were first explicitly defined in the 1940s, but they had already appeared in attempts to geometrically describe domains of holomorphy at the very beginning of several complex variables in the first decade of the 20th century. Since the 1960s, one of their most important roles has been as weights in a priori estimates for solving the Cauchy-Riemann equation. They are intimately related to the complex Monge-Amp`ere equation, the second partial differential equation of complex analysis. There is also a potential-theoretic aspect to plurisubharmonic functions, which is the subject of pluripotential theory.

In the early decades of the modern era of the subject, from the 1940s into the 1970s, the notion of a complex space took shape and the geometry of analytic varieties and holomorphic maps was developed. Also, three approaches to solving the Cauchy-Riemann equations were discovered and applied. First came a sheaf-theoretic approach in the 1950s, making heavy use of homological algebra. Hilbert space methods appeared in the early 1960s and integral formulas around 1970 through interaction with partial differential equations and harmonic analysis. The complex Monge-Amp`ere equation came to the fore in the late 1970s with Yau's solution of the Calabi conjectures and Bedford and Taylor's work on the Dirichlet problem.

Most current work in complex analysis and complex geometry can be seen as being focused on one or both of the two fundamental partial differential equations, Cauchy-Riemann and Monge-Amp`ere, in the setting of Euclidean space or more general complex manifolds. The past ten years have seen an increasing thrust towards extending both the theory and its applications to singular spaces, to almost complex manifolds, and to infinite-dimensional manifolds. These generalizations will be a major focus of the workshop.

Today, as before, complex analysis and complex geometry is a highly interdisciplinary field. The foundational work described above has been followed by a broad range of research at the interfaces with a number of other areas, such as algebraic geometry, functional analysis, partial differential equations, and symplectic geometry, to name a few. Complex analysts and complex geometers share a common toolkit, but find inspiration and open problems in many areas of mathematics. It would be beneficial to bring together a group of experts from diverse subfields to discuss recent results and work in progress and to share ideas on open questions. A single workshop cannot do justice to the breadth and depth of contemporary complex analysis and complex geometry. The organizers have chosen a coherent collection of interrelated topics for the workshop, representing, in their view, some of the most vibrant developments in the subject today. Each of the five topics will be briefly introduced in general terms, followed by a few recent highlights.

{bf 1. Analytic methods in complex algebraic geometry} are based on increasingly sophisticated ways of solving the Cauchy-Riemann equation (often also called the $barpartial$-equation) with $L^2$-estimates using plurisubharmonic weights in geometric settings. Yum-Tong Siu has long been a leader in this area. His announcement of an analytic proof of the finite generation of the canonical ring of a smooth complex projective variety of general type in October 2006 ({tt arXiv:math/0610740}) came on the heels of an algebraic proof by Birkar, Cascini, Hacon, and McKernan. This is a milestone in algebraic geometry.

Bo Berndtsson and Mihai Paun use analytic methods to obtain a nearly optimal criterion for the pseudoeffectivity of relative canonical bundles and give several applications in algebraic geometry ({tt arXiv:math/0703344}). Shigeharu Takayama uses analytic techniques, including multiplier ideal sheaves, to extend Siu's celebrated result on the invariance of plurigenera from the smooth case to the case of fibres with canonical singularities ({it J. Algebraic Geom.} {bf 16} (2007) 1--18). So far, there is no known algebraic proof of the full result.

Currents are differential forms with distribution coefficients; closed currents satisfying a certain positivity condition are objects of fundamental importance that generalize both K"ahler forms and analytic subvarieties. New work of Tien-Cuong Dinh and Nessim Sibony advances the basic theory of currents (intersections, pullbacks, etc.) and has many potential applications ({tt arXiv:math/0606248}, {tt arXiv:math/0703702}).

{bf 2. Pluripotential theory and the Monge-Amp`ere equation.} Pluripotential theory on compact K"ahler manifolds, based on the notion of a quasiplurisubharmonic function, has been developed by Vincent Guedj and Ahmed Zeriahi since 2004 (starting with {tt arXiv:math/0401302}). Whereas plurisubharmonic functions have a positive Levi form by definition and are constant on compact manifolds, quasiplurisubharmonic functions are allowed to have a negative Levi form down to a fixed lower bound and lead to a fruitful pluripotential theory in a compact setting. Guedj, Zeriahi, and Philippe Eyssidieux posted a major application of this theory in March 2006 ({tt arXiv:math/0603431}). They extended the work of Aubin and of Yau on the complex Monge-Amp`ere equation to certain singular settings and proved that the canonical model of a smooth complex projective variety of general type (proved to exist soon afterwards by Birkar et al. and by Siu --- this is equivalent to finite generation of the canonical ring) has a K"ahler-Einstein metric of negative Ricci curvature. This is only one example, albeit a very important one, of current work on the complex Monge-Amp`ere equation.

The highly nonlinear nature of the Monge-Amp`ere operator presents many challenges. Proper understanding of its maximal domain of definition in the local case of a domain in Euclidean space was obtained only recently in work of Zbigniew Bl ocki ({it Amer. J. Math.} {bf 128} (2006) 519--530). Surprisingly, very recent work of Guedj, Zeriahi, and Dan Coman has shown the domain of definition to be much larger in the global case of a compact K"ahler manifold ({tt arXiv:0705.4529}).

Coman and Evgeny Poletsky have derived new Bernstein, Bezout, and Markov inequalities using pluripotential theory and applied them to transcendental number theory ({it Invent. Math.} {bf 170} (2007) 103--145, {tt arXiv:math/0403420}). Their results have been generalized by Alexander Brudnyi ({tt arXiv:0708.3449}). In a series of papers, the earliest posted in 2002, Charles Favre and Mattias Jonsson have made a deep study of the singularities of plurisubharmonic functions and multiplier ideals in two dimensions using the novel concept of a tree of valuations (see Springer Lecture Notes in Mathematics, vol. 1853, 2004). In a new paper with Sebastien Boucksom, they have extended some of their work to higher dimensions ({tt arXiv:math/0702487}). A connection with probability theory appears in recent work of Thomas Bloom and Bernard Shiffman ({tt arXiv:math/0605739)} and of Robert Berman ({tt arXiv:math/0608226)}, who use various techniques of pluripotential theory to study zeros of random polynomials and, more generally, random sections of holomorphic line bundles.

{bf 3. The Cauchy-Riemann equation on singular spaces.} Existing methods for solving the Cauchy-Riemann equation are largely restricted to smooth spaces. Consequently, the central problem of classical several complex variables, the Levi problem, which asks whether Steinness is a local property and was solved for manifolds decades ago, is still open for singular spaces. Progress in this area has been slow and difficult. Recently, John Erik Fornae ss, Nils O vrelid, and Sophia Vassiliadou have been able to solve the Cauchy-Riemann equations with $L^2$-estimates in certain singular settings ({tt arXiv:math/0401171}, {tt arXiv:math/0610569}).

{bf 4. Almost complex geometry.} In a seminal paper of 1985, Gromov introduced almost complex structures and pseudoholomorphic curves into symplectic topology. Interaction between complex geometry and symplectic geometry began in earnest with the work of Sergey Ivashkovich and Vsevolod Shevchishin in the late 1990s ({tt arXiv:math/9912046}). There is now a growing body of work concerned with extending concepts and results from complex analysis and complex geometry to the almost complex case. Often the non-integrable case requires new methods that shed light on the integrable case. Notable new work includes a paper by Bernard Coupet, Alexander Tumanov, and Alexander Sukhov on proper pseudoholomorphic discs ({tt arXiv:0704.0124}), a long paper on fundamentals of local almost complex geometry by Coupet, Sukhov, and Herv'e Gaussier ({tt arXiv:math/0701576}), a paper by Xianghong Gong and Jean-Pierre Rosay on removable singularities of pseudoholomorphic maps ({tt arXiv:0708.1726}), and a paper by Ivashkovich and Shevchishin on almost complex structures that are merely Lipschitz ({tt arXiv:0707.0771})

{bf 5. Infinite-dimensional complex geometry.} Complex analysis in infinite dimensions languished outside the mainstream until L'aszl'o Lempert commenced a major research program in the mid-1990s. Generalized loop spaces (spaces of smooth maps from a compact smooth manifold into a finite-dimensional complex manifold) are examples of infinite-dimensional complex manifolds; their importance in physics provides strong motivation for Lempert's program. Fundamental notions, including Dolbeault cohomology, coherence of analytic sheaves, and holomorphic approximation, have been brought into an infinite-dimensional setting by Lempert, in part with coauthors Endre Szab'o and Ning Zhang ({tt arXiv:math/0403405}, {tt arXiv:math/0507449}, {tt arXiv:math/0507549}). Imre Patyi has studied the Oka principle for infinite-dimensional complex manifolds ({tt arXiv:math/0509557}).

{bf Timeliness and community interest.} Why is this a timely proposal for 2009? Most of the work we have cited is brand new, posted in 2006 or 2007, and not yet in print. We want to hold an international workshop on these developments as soon as possible, and the earliest it can be, at least if we want to avail ourselves of the excellent facilities and inspiring natural setting of BIRS, is 2009.

There is strong community interest in the workshop proposed here. Nine days before submitting this proposal, we sent out an e-mail to a group of people, including most of the authors cited above, and asked: {it If the workshop goes ahead and is scheduled at a time suitable for you, would you be interested in attending it?} Within a week, 37 people had replied and expressed an interest in the workshop. Thus, the 41 people on our list of possible participants have all confirmed their interest in the workshop.