“…The procedure of obtaining the system for ϕ , ψ and θ is quite standard (one has to calculate the limit for as ( x , y ) tends to a point of the boundary, cf Romero Rodríguez and Zhevandrov and Garibay and Zhevandrov). Using the formulas for the Fourier transform of the function K 0 and its derivative, one finally comes to the following system for , and θ (we omit these tedious calculations): where …”