Alur, RajeevKumar, VirajMadhusudan, P.Viswanathan, Mahesh2023-05-222023-05-222005-07-112005-06-06https://repository.upenn.edu/handle/20.500.14332/6219We study congruences on words in order to characterize the class of visibly pushdown languages (VPL), a subclass of context-free languages. For any language L, we define a natural congruence on words that resembles the syntactic congruence for regular languages such that this congruence is of finite index if, and only if, L is a VPL. We then study the problem of finding canonical minimal deterministic automata for VPLs. Though VPLs in general do not have unique minimal automata, we consider a subclass of VPAs called k-module single-entry VPAs that correspond to programs with recursive procedures without input parameters, and show that the class of well-matched VPLs do indeed have unique minimal k-module single-entry automata. We also give a polynomial time algorithm that minimizes such k-module single-entry VPAs.The original publication is available at www.springerlink.comCPS Formal MethodsCPS TheoryPresentation