dc gain transfer function
leotim was close, albeit off by one:
For discrete LTI systems, most transfer functions may be expressed as a ratio of two polynomials in z.
The numerator polynomial has the zeroes of the transfer function as its roots, whereas the denominator has the poles.
The DC response exp(j0) may be readily solved by substituting z=1 into the polynomials.
Again ... this is for discrete LTI systems ...
Continuous systems solve differential equations, and, as such, are subject to boundary conditions for their complete solution.