Sunitt Shantanu Digamber Fulari it's just negatives powers of z (z^{-i}, i = 0, 1, 2, ..., N).

When I use the definition, my result of G(z) it's very confused, because it's a polynomial with hundreds of terms (I would like to simplify this). My partial solution it's to use the transfer function of a 1st order system. I obtain a and b of g(t) = a*exp(-b*(t)) using fit( ) function and I obtain the transfer function of this.