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