I want to solve the following PDE with boundary conditions, but unfortunately I do not know the meaning of _F1 and _F2 in this stuation. Would you please help me to find these arbitrary functions?
> PDE := diff(U(y, z), y, y)+diff(U(y, z), z, z) = 0;
> sol := pdsolve(PDE);
U(y, z) = _F1(z - I y) + _F2(z + I y)
What's the meaning of _F1 and _F2 in this situation? How can I find them?