Algebro-geometric approach to the Ernst equation I. Mathematical Preliminaries
1. Introduction. It is well known that methods of algebraic geometry and, in particular, Riemann surface techniques are well suited for the solution of nonlinear integrable equations. For instance, for nonlinear evolution equations, so called 'finite gap' solutions have been found by the help of these methods. In 1989 Korotkin  succeeded in applying these techniques to the Ernst equation, which is equivalent to Einstein's vacuum equation for axisymmetric stationary fields. But, the Ernst equation is not an evolution equation and, due to this fact, one is in this case usually confronted with boundary value problems which have not been considered there. On the other hand, Neugebauer and Meinel  were able to transform the boundary value problem for the rigidly rotating disk of dust into a scalar Riemann-Hilbert problem on a hyperelliptic Riemann surface and gave the solution to this problem in terms of theta functions. The methods they used were suited to the particular problem and one may ask to which extent algebro-geometric methods are useful for the solution of boundary value problems of the Ernst equation. In order to tackle this problem one should at first develop the Riemann-Hilbert technique on Riemann surfaces in detail and then apply this method in order to find solutions to the Ernst equation. The first of these two papers is devoted to the brief introduction into Riemann surface techniques (for a more detailed exposition see the cited literature). The second paper shows how the developed methods apply to the Ernst equation.