Describing a system that is causal and stable and has an inverse system that is also causal and stable. A system S converting input x to output y has an inverse if there is another system S⁻¹ that takes y as input and produces x as output; if y=S(x) holds, then x=S⁻¹(y) must also hold. Such systems have zeros and poles that are strictly inside the unit circle if discrete, and strictly inside the left-hand s-plane if continuous.