This open access two-volume set constitutes the proceedings of the 27th International Conference on Tools and Algorithms for the Construction and Analysis of Systems, TACAS 2021, which was held during March 27 – April 1, 2021, as part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2021.

Game Theory.- A Game for Linear-time - Branching-time Spectroscopy.- On Satisficing in Quantitative Games.- Quasipolynomial Computation of Nested Fixpoints.- SMT Verification.- A Flexible Proof Format for SAT Solver-Elaborator Communication.- Generating Extended Resolution Proofs with a BDD-Based SAT Solver.- Bounded Model Checking for Hyperproperties.- Counterexample-Guided Prophecy for Model Checking Modulo the Theory of Arrays.- SAT Solving with GPU Accelerated Inprocessing.- FOREST: An Interactive Multi-tree Synthesizer for Regular Expressions.- Probabilities.- Finding Provably Optimal Markov Chains.- Inductive Synthesis for Probabilistic Programs Reaches New Horizons.- Analysis of Markov Jump Processes under Terminal Constraints.- Multi-objective Optimization of Long-run Average and Total Rewards.- Inferring Expected Runtimes of Probabilistic Integer Programs Using Expected Sizes.- Probabilistic and Systematic Coverage of Consecutive Test-Method Pairs for Detecting Order-DependentFlaky Tests.- Timed Systems.- Timed Automata Relaxation for Reachability.- Iterative Bounded Synthesis for Efficient Cycle Detection in Parametric Timed Automata.- Algebraic Quantitative Semantics for Efficient Online Temporal Monitoring.- Neural Networks.- Synthesizing Context-free Grammars from Recurrent Neural Networks.- Automated and Formal Synthesis of Neural Barrier Certificates for Dynamical Models.- Improving Neural Network Verification through Spurious Region Guided Refinement.- Analysis of Network Communication Resilient Capacity-Aware Routing.- Network Traffic Classification by Program Synthesis. 

This open access two-volume set constitutes the proceedings of the 27th International Conference on Tools and Algorithms for the Construction and Analysis of Systems, TACAS 2021, which was held during March 27 – April 1, 2021, as part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2021. The conference was planned to take place in Luxembourg and changed to an online format due to the COVID-19 pandemic.The total of 41 full papers presented in the proceedings was carefully reviewed and selected from 141 submissions. The volume also contains 7 tool papers; 6 Tool Demo papers, 9 SV-Comp Competition Papers.The papers are organized in topical sections as follows:Part I: Game Theory; SMT Verification; Probabilities; Timed Systems; Neural Networks; Analysis of Network Communication.Part II: Verification Techniques (not SMT); Case Studies; Proof Generation/Validation; Tool Papers; Tool Demo Papers; SV-Comp Tool Competition Papers.

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