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This book constitutes the proceedings of the 21st International Conference on Descriptional Complexity of Format Systems, DCFS 2019, held in Kosice, Slovakia, in July 2019.

The 18 full papers presented in this volume were carefully reviewed and selected from 25 submissions. The book also contains 4 invited talks. They deal with all aspects of descriptional complexity and costs of description of objects in various computational models, such as Turing machines, pushdown automata, finite automata, grammars, and others.

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A General Framework for Sequential Grammars with Control Mechanisms.- Low-Complexity Tilings of the Plane.- Union-Freeness, Deterministic Union-Freeness and Union-Complexity.- Limited Automata: Properties, Complexity and Variants.- Nondeterministic Right One-Way Jumping Finite Automata.- State Complexity of Single-Word Pattern Matching in Regular Languages.- Square, Power, Positive Closure, and Complementation on Star-Free Languages.- Descriptional Complexity of Matrix Simple Semi-Conditional Grammars.- Regulated Tree Automata.- Generalized de Bruijn Words and the State Complexity of Conjugate Sets.- The Syntactic Complexity of Semi-Flower Languages.- Limited Nondeterminism of Input-Driven Pushdown Automata Decidability and Complexity.- Computability on Quasi-Polish Spaces.- NFA-to-DFA Trade-Off for Regular Operations.- State Complexity of Simple Splicing.- Nondeterminism Growth and State Complexity.- Descriptional Complexity of Iterated Uniform Finite State Transducers.- On Classes of Regular Languages Related to Monotone WQOs.- State Complexity of GF(2)-Concatenation and GF(2)-Inverse on Unary Languages.- Pushdown Automata and Constant Height: Decidability and Bounds.- On the Decidability of Finding a Positive ILP-Instance in a Regular Set of ILP-Instances.- How Does Adiabatic Quantum Computation Fit into Quantum Automata Theory.

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