Dear Maple community, 
I am struggling with the numerical solution of the complex (linear) partial differential equation to obtain results. For example, I see such as this statement "Error, (in pdsolve/numeric/plot) unable to compute solution for t<HFloat(0.0): signum is not differentiable at complex values" pops up. I did my homework but I didn't get anything. I will be happy if someone could give me some suggestions or advice. Thank you in advance. 

Hello Everyone,

May I ask you about this  "Error,   (in pdsolve/numeric/process_PDEs)  number of dependent variables and number of PDE must be the same". Does anyone have idea about solving linear instability equation (flow inside pipe, oscillating flow) ?

Thank you,