Hi,

I am a beginner and am trying to solve the following PDE but get an error. 

PDE:=-diff(f(x1,x2),x1,x1)-diff(f(x1,x2),x2,x2) = a^2*f(x1,x2);

IBC := {D[1](f)(0,x2) = 0, D[1](f)(L1,0) = 0, D[1](f)(x1,0) = 0, D[1](f)(x1,L2) = 0};

ans := pdsolve(IBC,PDE);

"Error, (in pdsolve/BC) unable to handle derivatives in BCs involving arbitrary

 functions _Fn(...)"

Please guide me.