[mg86118] I couldn't solve a pde problem with boundary values in mathematica

[atepe <aysegul.tepe at yahoo.com>]

Hi,

I am trying to solve the following partial differential equation with 4 boundary conditions with mathematica. But it doesn't work. 
 
sigma is a constant in here.
**************************** 
Simplify[DSolve[
t[y, z] - \[Gamma]^2*(D[t[y, z], {y, 2}] + 
D[t[y, z], {z, 2}]) == \[Sigma], t, {y, z}]]
 
The boundary conditions are:
t(y,0)=\sigma
t(y,L)=\sigma
t(-h/2,Z)=0
t(h/2,Z)=0
 
Does anyone know how to solve the problem?

Thanks for advance