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This book constitutes the proceedings of the 19th International Conference on Descriptional Complexity of Formal Systems, DCFS 2017, held in Milano, Italy, in July 2017. The 20 full papers presented together with 4 invited talks were carefully reviewed and selected from 26 submissions.Descriptional Complexity is a field in Computer Science that deals with the size of all kinds of objects that occur in computational models, such as turing machines, finite automata, grammars, splicing systems and others. The topics of this conference are related to all aspects of descriptional complexity.

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Sensing as a Complexity Measure.- Avoiding Overlaps in Pictures.- Descriptional Complexity and Operations - Two non-Classical Cases.- Applications of Transducers in Independent Languages, Word Distances, Codes.- On the Degree of Nondeterminism of Tree Adjoining Languages and Head Grammar Languages.- On the Average Complexity of Strong Star Normal Form.- Most Complex Non-Returning Regular Languages.- Uncountable realtime probabilistic classes.- A Parametrized Analysis of Algorithms on Hierarchical Graphs.- Graph-Controlled Insertion-Deletion Systems Generating Language Classes Beyond Linearity.- Computational Completeness of Networks of Evolutionary Processors with Elementary Polarizations and a Small Number of Processors.- Recognizing Union-Find trees built up using union-by-rank strategy is NP-complete.- Self-attraction removal from oritatami systems.- One-Time Nondeterministic Computations.- Kuratowski Algebras Generated by Factor-, Subword-, and Suffix-Free Languages.- Branching Measures and Nearly Acyclic NFAs.- Square on Deterministic, Alternating, and Boolean Finite Automata.- A Pumping Lemma for Ordered Restarting Automata.- Concise Representations of Reversible Automata.- State Complexity of Unary SV-XNFA with Different Acceptance Conditions.- Reset Complexity of Ideal Languages Over a Binary Alphabet.- 2-state 2-symbol Turing machines with periodic support produce regular sets.- State Complexity of Suffix Distance.- The quotient operation on input-driven pushdown automata.

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