Well, in the end I'm interested in G(r,r'), which has to look like this G(r,r')=1/(r-r')+f(r,r') and for which I know the 6 boundary conditions. One could imagine G inside a cube, and it has to be 0 at the cube's walls. So the catesian coordinates are: x-x', y-y', z-z' and I expressed it in the vector r, just to make it simpler to write down: r=sqrt((x-x')^2+(y-y')^2+(z-z')^2) An examaple: It is easy if I only had one wall, maybe somewhere along the z-axis : Then: f(r,r')=-1/(r-r_wall) Now: G(r,r_wall)=1/(r-r_wall)-1/(r-r_wall)=0 This r_wall would be from my first bounday condition G(r,r_wall)=G(x,y,a)=0 --> r_wall=sqrt((x-x')^2+(y-y')^2+(z-a)^2) I do all that to find the Greenfunction for a specific problem in Electrostatics. I hope I make more sense now. Thanks