Question: PDE _F1 _F2



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;

> with(PDEtools);

> 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?

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