Plurisubharmonic function

In mathematics, plurisubharmonic functions form an important class of functions used in complex analysis. On a Kahler manifold,plurisubharmonic functions form a subset of the subharmonic functions. However, unlikesubharmonic functions (which are defined on a
Riemannian manifold) plurisubharmonic functions can be defined in full generality on complex spaces.

Formal definition

A function :$f\; colon\; G\; o\; \{mathbb\{Rcup\{-infty\}$, with "domain" $G\; subset\; \{mathbb\{C^n$ is called plurisubharmonic if it is upper semi-continuous, and for every complex line

:$\{\; a\; +\; b\; z\; mid\; z\; in\; \{mathbb\{C\; \}subset\; \{mathbb\{C^n$ with $a,\; b\; in\; \{mathbb\{C^n$

the function $z\; mapsto\; f(a\; +\; bz)$ is a subharmonic function on the set

:$\{\; z\; in\; \{mathbb\{C\; mid\; a\; +\; b\; z\; in\; G\; \}.$

In "full generality", the notion can be defined on an arbitrary complex manifold or even a complex space $X$ as follows. An upper semi-continuous function :$f\; colon\; X\; o\; \{mathbb\{R\; cup\; \{\; -\; infty\; \}$is said to be plurisubharmonic if and only if for any holomorphic map$varphicolonDelta\; o\; X$ the function:$fcircvarphi\; colon\; Delta\; o\; \{mathbb\{R\; cup\; \{\; -\; infty\; \}$is subharmonic, where $Deltasubset\{mathbb\{C$ denotes the unit disk.

Differentiable plurisubharmonic functions

If $f$ is of (differentiability) class $C^2$, then $f$ is plurisubharmonic, if and only if the hermitian matrix $L\_f=(lambda\_\{ij\})$, called Levi matrix, withentries

:

is positive semidefinite.

Equivalently, a $C^2$-function "f" is plurisubharmonic if and only if is a positive (1,1)-form.

History

Plurisubharmonic functions were defined in 1942 by
Kiyoshi Oka K. Oka, "Domaines pseudoconvexes," Tohoku Math. J. 49 (1942), 15-52.] and Pierre Lelong. [ P. Lelong, "Definition des fonctions plurisousharmoniques," C. R. Acd. Sci. Paris 215 (1942), 398-400.]

Properties

*The set of plurisubharmonic functions form a convex cone in the vector space of semicontinuous functions, i.e.:* if $f$ is a plurisubharmonic function and $c>0$ a positive real number, then the function $ccdot\; f$ is plurisubharmonic,:* if $f\_1$ and $f\_2$ are plurisubharmonic functions, then the sum $f\_1+f\_2$ is a plurisubharmonic function.
*Plurisubharmonicity is a "local property", i.e. a function is plurisubharmonic if and only if it is plurisubharmonic in a neighborhood of each point.
*If $f$ is plurisubharmonic and $phi:mathbb\{R\}\; omathbb\{R\}$ a monotonically increasing, convex function then $phicirc\; f$ is plurisubharmonic.
*If $f\_1$ and $f\_2$ are plurisubharmonic functions, then the function $f(x):=max(f\_1(x),f\_2(x))$ is plurisubharmonic.
*If $f\_1,f\_2,...$ is a monotonically decreasing sequence of plurisubharmonic functionsthen so is $f(x):=lim\_\{n\; oinfty\}f\_n(x)$.
*Every continuous plurisubharmonic function can be obtained as a limit of monotonically decreasing sequence of smooth plurisubharmonic functions. Moreover, this sequence can be chosen uniformly convergent. [R. E. Greene and H. Wu, "$C^infty$-approximations of convex, subharmonic, and plurisubharmonic functions", Ann. Scient. Ec. Norm. Sup. 12 (1979), 47-84.]
*The inequality in the usual semi-continuity condition holds as equality, i.e. if $f$ is plurisubharmonic then

: $limsup\_\{x\; o\; x\_0\}f(x)\; =f(x\_0)$

(see limit superior and limit inferior for the definition of "lim sup").

* Plurisubharmonic functions are subharmonic, for any Kähler metric.

*Therefore, plurisubharmonic functions satisfy the maximum principle, i.e. if $f$ is plurisubharmonic on the connected domain $D$ and

: $sup\_\{xin\; D\}f(x)\; =f(x\_0)$

for some point $x\_0in\; D$ then $f$ is constant.

Applications

In complex analysis, plurisubharmonic functions are used to describe pseudoconvex domains, domains of holomorphy and Stein manifolds.

Oka theorem

The main geometric application of the theory of plurisubharmonic functions is the famous theorem proven by Kiyoshi Oka in 1942. K. Oka, "Domaines pseudoconvexes," Tohoku Math. J. 49 (1942), 15-52.]

A continuous function $f:;\; M\; mapsto\; \{Bbb\; R\}$is called "exhaustive" if the preimage $f^\{-1\}(]\; -infty,\; c]\; )$is compact for all $cin\; \{Bbb\; R\}$. A plurisubharmonicfunction "f" is called "strongly plurisubharmonic"if the form is positive, for some Kähler form$omega$ on "M".

Theorem of Oka: Let "M" be a complex manifold,admitting a smooth, exhaustive, strongly plurisubharmonic function.Then "M" is Stein. Conversely, any
Stein manifold admits such a function.

References

* Steven G. Krantz. Function Theory of Several Complex Variables, AMS Chelsea Publishing, Providence, Rhode Island, 1992.
* Robert C. Gunning. Introduction to Holomorphic Functions in Several Variables, Wadsworth & Brooks/Cole.

Notes