I have a couple of questions about the theory about how to tell whether a system is casual and FIR or IIR. First point is about a casual system. Is it true that all FIR systems are casual? How can you tell if an IIR system is casual? I am I correct in thinking that a FIR system only has a numerator and IIR system has both a numerator and denominator? For example: h(n) = u(n+1) -u(n-1) and h(n) = 5^n . U(-n) Are both of these FIR systems?