Knot thickness

In mathematics, more particularly in complex geometry, algebraic geometry and complex analysis, a positive current is a positive (n-p,n-p)-form over an n-dimensional complex manifold, taking values in distributions.

For a formal definition, consider a manifold M. Currents on M are (by definition) differential forms with coefficients in distributions. ; integrating over M, we may consider currents as "currents of integration", that is, functionals

$\eta \mapsto \int _{M}\eta \wedge \rho$

on smooth forms with compact support. This way, currents are considered as elements in the dual space to the space $\Lambda _{c}^{*}(M)$ of forms with compact support.

Now, let M be a complex manifold. The Hodge decomposition $\Lambda ^{i}(M)=\bigoplus _{p+q=i}\Lambda ^{p,q}(M)$ is defined on currents, in a natural way, the (p,q)-currents being functionals on $\Lambda _{c}^{p,q}(M)$.

A positive current is defined as a real current of Hodge type (p,p), taking non-negative values on all positive (p,p)-forms.

Characterization of Kähler manifolds

Using the Hahn–Banach theorem, Harvey and Lawson proved the following criterion of existence of Kähler metrics.

Theorem: Let M be a compact complex manifold. Then M does not admit a Kähler structure if and only if M admits a non-zero positive (1,1)-current $\Theta$ which is a (1,1)-part of an exact 2-current.

Note that the de Rham differential maps 3-currents to 2-currents, hence $\Theta$ is a differential of a 3-current; if $\Theta$ is a current of integration of a complex curve, this means that this curve is a (1,1)-part of a boundary.

When M admits a surjective map $\pi :\;M\mapsto X$ to a Kähler manifold with 1-dimensional fibers, this theorem leads to the following result of complex algebraic geometry.

Corollary: In this situation, M is non-Kähler if and only if the homology class of a generic fiber of $\pi$ is a (1,1)-part of a boundary.

References

• Phillip Griffiths and Joseph Harris (1978), Principles of Algebraic Geometry, Wiley. ISBN 0-471-32792-1
• J.-P. Demailly, $L^2$ vanishing theorems for positive line bundles and adjunction theory, Lecture Notes of a CIME course on "Transcendental Methods of Algebraic Geometry" (Cetraro, Italy, July 1994)

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