# Problem 2.28, HW3, ECE301, Summer 2008

Determine if each system is causal and stable.

**A**

h[n] = (1/5)$ ^n $ u[n]

For n < 0 h[n] = 0 therefore h[n] is causal.

$ \Sigma_{n=0}^\infty $ (1/5)$ ^n $ < $ \infty $ since lim$ _{n->\infty} $ = 0

The system is both causal and stable.

**B**

h[n] = $ (0.8)^n $ u[n+2]

Since u[n+2] = 1 for n >= -2 and 0 for n < -2 the system is not causal because h[n] $ \neq $ 0 for t < 0.

$ \Sigma_{n = -2}^\infty $ $ (0.8)^n $ < $ \infty $ since $ lim_{n->\infty} (0.8)^n = 0 $, the system is stable.

The system is not causal and stable.

**D**

h[n] = 5$ ^n $u[3-n]

Since u[3-n] = 1 for n <= 3 and 0 for n > 3, h[n] $ \neq $ 0 for t < 0.

$ \Sigma_{-\infty}^\infty 5^n u[3-n] = \Sigma_{-\infty}^3 5^n < \infty $, therefore the system is stable.

This system is stable but not causal.