$ y[n] = \sum_{n=-\infty}^{\infty}x[n] \; \; $ (DT integral)
$ h[n] = \sum_{n=-\infty}^{\infty}\delta [n] = u[n] $
$ H(z) = \sum_{m=-\infty}^{\infty}h[m] e^{-j \omega m} $
