Design a PDA for a^n b^(2n) | Automata Lecture 06

Learn how to design a Pushdown Automaton (PDA) for the language a^n b^(2n), step by step. In this Theory of Automata lecture, you will turn the “twice as many b's” rule into a correct stack-based PDA. Lecture 03 in the FacultyLearn Theory of Automata and Formal Languages playlist explains a standard undergraduate PDA-construction problem: L = {a^n b^(2n)}. The challenge is not simply recognising a's followed by b's; the PDA must verify that every a is matched by exactly two b's, while rejecting incorrect orderings and incorrect counts. You will learn: • how to read the formal language and identify the counting relationship; • why a finite automaton cannot solve this dependency; • the core stack invariant: push two markers for every a, then pop one marker for every b; • how to divide the construction into an a-reading phase and a b-reading phase; • how to define acceptance, including the role of the bottom-of-stack marker; • how to trace a valid string and identify common exam mistakes. This problem-solving lecture is for undergraduate Computer Science students, self-learners, international students, and exam-preparation learners studying Automata Theory, Formal Languages, or Computation Theory. It is especially useful when you need to move from an informal language description to a state-and-stack design that you can explain clearly in an assignment or exam. For structured learning resources and mentoring/academic support: Instructor: M. Imran Shafi Instructor profile: [INSTRUCTOR PROFILE LINK] Subscribe to FacultyLearn | University CS Lectures for university-level Computer Science lessons taught with clear, rigorous explanations. In the comments, share the string you would use to test this PDA first, and explain whether it should be accepted or rejected. Copyright and learning-use notice: This original FacultyLearn lecture, its slides, diagrams, narration, and supporting materials are protected content. Do not re-upload, redistribute, or reuse substantial portions without written permission. You may reference the concepts for personal study with proper attribution. #FacultyLearn #AutomataTheory #PushdownAutomata #FormalLanguages #ComputerScience