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This book constitutes the refereed proceedings of the 21st International Conference on Implementation and Application of Automata, CIAA 2016, held in Seoul, South Korea, in July 2016.
The 26 revised full papers presented were carefully reviewed and selected from 49 submissions. The papers cover a wide range of topics including characterizations of automata, computing distances between strings and languages, implementations of automata and experiments, enhanced regular expressions, and complexity analysis.

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Decision Problems for Finite Automata over Infinite Algebraic Structures.- The Degree of Irreversibility in Deterministic Finite Automata.- Deterministic Stack Transducers.- Computing the Expected Edit Distance from a String to a PFA.- Derived-Term Automata of Multitape Rational Expressions.- Solving Parity Games Using an Automata-Based Algorithm.- Ternary Equational Languages.- Problems on Finite Automata and the Exponential Time Hypothesis.- A Practical Algorithm for the Uniform Membership Problem of Labeled Multidigraphs of Tree-Width 2 for Spanning Tree Automata.- A Practical Simulation Result for Two-Way Pushdown Automata.- Nondeterministic Complexity of Operations on Closed and Ideal Languages.- On Bounded Semilinear Languages, Counter Machines, and Finite-Index ET0L.- Kuratowski Algebras Generated by Prefix-Free Languages.- A Logical Characterization of Small 2NFAs.- Experiments with Synchronizing Automata.- Implementation of Code Properties via Transducers.- On Synchronizing Automata and Uniform Distribution.- Looking for the Pairs that are Hard to Separate: A Quantum Approach.- Prefix Distance Between Regular Languages.- Complexity of Sets of Two-Dimensional Patterns.- The Complexity of Fixed-Height Patterned Tile Self-Assembly.- Derivative-Based Diagnosis of Regular Expression Ambiguity.- Regular Approximation of Weighted Linear Nondeleting Context-Free Tree Languages.- Derivatives for Enhanced Regular Expressions.- Weighted Restarting Automata as Language Acceptors.- Enhancing Practical TAG Parsing Efficiency by Capturing Redundancy.- Analyzing Matching Time Behavior of Backtracking Regular Expression Matchers by Using Ambiguity of NFA.

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