## Geodesic length functions and the Nielsen problem.(English)Zbl 0616.53039

In view of the author’s result establishing the noncompleteness of the Weil-Petersson metric for the Teichmüller space $$T_{g,n}$$, the geodesical convexity of $$T_{g,n}$$ was an open problem. The present paper is devoted to this question. Roughly speaking, the author shows that the geodesic structure in the Weil-Petersson geometry is quite similar to the one of a complete Riemannian metric of negative curvature.
The basic idea in the author’s approach is to substitute for completeness the use of the geodesic length functions introduced by Fricke-Klein. The paper contains a comprehensive exposition of Teichmüller theory and the Weil-Petersson geometry by means of harmonic Beltrami differentials. Regarding the geometric properties of $$T_{g,n}$$ the author proves that every pair of points is joined by a unique geodesic, the exponential map is a homeomorphism and the Weil-Petersson distance is measured along geodesics. As applications he gives new proofs that $$T_{g,n}$$ is Stein and of the Nielsen realization problem.
Reviewer: D.Motreanu

### MSC:

 53C22 Geodesics in global differential geometry 30F30 Differentials on Riemann surfaces 30F35 Fuchsian groups and automorphic functions (aspects of compact Riemann surfaces and uniformization)
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