hi, i am trying to solve a PDE f(x,z,t) with mixed boundary conditions, while Maple just gives u(x,z,t)=0 which is incorrect, so i believe somewhere must be wrong, someone has an idea?  

Governing equation: ,    0<x<M, 0<z<L, t>0

IC: heat source is a point at (0, z0):  u(x,z,0)=c*dirac(x-0)*dirac(z-z0) , where c is a temperature at t=0.

boundarys of domain are cauchy boundaries: du(0,z,t)/dx=0;  du(M,z,t)/dx=0; du(x,0,t)/dz=0;  du(x,L,t)/dz=0

the code is: 

PDE := diff(u(x, z, t), t) = a*(diff(u(x, z, t), x, x))+b*(diff(u(x, z, t), z, z));
IBC := u(x, z, 0) = c*Dirac(x-0)*Dirac(z-z0), (D[1](u))(0, z, t) = 0, (D[1](u))(M, z, t) = 0, (D[2](u))(x, 0, t) = 0, (D[2](u))(x, L, t) = 0;

pdsolve([PDE, IBC], u(x, z, t)) assuming 0<x<M, 0<z<L

thanks in advance!!