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Published online by Cambridge University Press: 15 December 2002
We introduce a notion of category with feedback-with-delay, closely relatedto the notion of traced monoidal category, and show that the Circconstruction of [15] is the free category with feedback on a symmetricmonoidal category. Combining with the Int construction ofJoyal et al. [12] we obtain a description of the free compact closedcategory on a symmetric monoidal category. We thus obtain a categoricalanalogue of the classical localization of a ring with respect to amultiplicative subset. In this context we define a notion of fixed-pointsemantics of a category with feedback which is seen to include a variety ofclassical semantics in computer science.