Originally Posted by

**shawsend** That's confusing. f(s), the transform, is a function of a complex variable s=x+iy and is itself complex. The command:

Plot3D[Abs[f(x+iy)],{x,-3,3},{y,-3,3}] is implicitly taking z=Abs(H(s)), x is set equal to the real component of the complex variable x+iy, and y is set equal to the imaginary component.

What does $\displaystyle y-(\sigma-j\omega)\omega$ mean? Are you asking to set $\displaystyle y=(\sigma-j\omega)\omega$ with $\displaystyle j=\sqrt{-1}$? In that case y would itself be complex and you can't plot a complex variable directly. Need to take it's real part, imaginary part or absolute part.