Introduction to Weak Methods in Theorem Proving 13.1 The General Problem.pdf

Introduction to Weak Methods in Theorem Proving 13.1 The General Problem.pdf

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Introduction to Weak Methods in Theorem Proving 13.1 The General Problem

1Automated Reasoning 13 13.0 Introduction to Weak Methods in Theorem Proving 13.1 The General Problem Solver and Difference Tables 13.2 Resolution Theorem Proving 13.3 PROLOG and Automated Reasoning 13.4 Further Issues in Automated Reasoning 13.5 Epilogue and References 13.6 Exercises 2 Chapter Objective ? Learn about general-purpose theorem proving in predicate calculus. 3 The problem ? Given: a knowledge base (a set of sentences) ? Prove: a sentence Formally, ? Given: a Knowledge Base (KB), a sentence α ? Show whether: KB |= α (does KB entail α ? Or does α follow from KB ?) 4 The tool ? Modus ponens KB: p → q p question: q answer: yes {p → q, p} |= {q} ? We can form arbitrarily long “chains” of inference to prove a sentence ? We can use forward or backward reasoning 5Example ? If Mary goes to a party, Jane also does. If Jane goes to a party, she cannot study. If Jane cannot study, she fails. Mary went to a party. ? Can we prove: Jane will fail. 6 Example ? If Mary goes to a party, Jane also does. M J If Jane goes to a party, she cannot study. J C If Jane cannot study, she fails. C F Mary went to a party. M ? Can we prove: Jane will fail. F Does {M → J, J → C, C → F, M} entail {F}? 7 Example 1. M → J 2. J → C 3. C → F 4. M Modus ponens on 1 and 4: 5. J Modus ponens on 2 and 5: 6. C Modus ponens on 3 and 6: 7. F proven! 8 Another tool ? Modus tollens KB: p → q ?q entails ? p. ? So, a theorem proving process involves applying such rules until the desired sentence is proven. ? We call this a “proof” because the rules we use are sound (correct). 9Using modus ponens ? solves a lot of practical problems and is fairly efficient in terms of “searching” for a proof. ? Unfortunately, fails to prove some sentences which should be entailed by a KB (it is incomplete) 10 Example If Mary goes to the party, Jane also will. M J If Mary does not go to the party, Jane will. ?M J { M → J, ?M → J} should entail {J} because either M is true, or ?M is true and either way J

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