Hello, I know that Finite Impulse Response (FIR) filters can be equivalently expressed as a convolution. The effect of convolution in frequency domain is well known. In conclusion it is easy to make sense of FIR filters. My questions are: - can also Infinite Impulse Response (IIR) filters be given a similar interpretation? Are they convolution of some sort? - There exists some digital IIR filters which manipulate the frequencies of the signal (e.g. low-pass, hi-pass, etc...). How can one design an IIR filter which does specific operations to frequencies if we don't know how to handle it mathematically? Alternatively, how can you prove that a low-pass IIR filter in time-domain, does indeed filter frequencies?