# 011- logic puzzles and modal logic

This chapter is mainly about logic puzzles and modal logic, which mainly includes modal logics,closure properties in modal logic,specific modal logics,the modal logics S4 and S5,axioms and their assumptions,idealized concept of knowledge and so on.

1.Logic Puzzles and Modal Logic

2.Closure properties in modal logic

3.Closure properties in modal logic

4.Specific modal logics Specific modal logics are specified by giving formula schemes , which are then called axioms, and considering the corresponding closure as defined above. Note that by definition , K is an axiom of any such logic.

5.The Modal Logics S4 and S5 The logic S4, or KT4 , is characterized by axioms T and 4 (and K). The logic S5, or KT45 , is characterized by axioms T, 4, and 5 (and K). The latter logic is used to reason about knowledge and  is read as “I know  ."

6.Axioms and their assumptions

7.Idealized concept of knowledge The logic S5 thus formalizes an idealized concept of knowledge , different from human knowledge.

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9.The Wise-Men Puzzle There are three wise men, three red hats, and two white hats . The king puts a hat on each of the wise men in such a way that they are not able to see their own hat. He then asks each one in turn whether he knows the color of his hat. The first man says he does not know. The second man says he does not know either. What does the third man say?

10.The Muddy-Children Puzzle There is a variation on the wise-men puzzle in which the questions are not asked sequentially, but in parallel. A group of children is playing in the garden and some of the children, say k of them, get mud on their foreheads. Each child can see the mud on others only. Note that if k &gt; 1, then every child can see another with mud on its forehead. Now consider two scenarios:

11.Now consider two scenarios: The father repeatedly asks “Does any of you know whether you have mud on your forehead?". All children answer “no " the first time, and continue to answer “no " to repetitions of the same question. The father tells the children that at least one of them is muddy and repeatedly asks “Does any of you know whether you have mud on your forehead ?". This time, after the question has been asked k times, the k muddy children will answer ”yes ."

12.Now consider two scenarios: The father repeatedly asks “Does any of you know whether you have mud on your forehead?". All children answer “no " the first time, and continue to answer “no " to repetitions of the same question. The father tells the children that at least one of them is muddy and repeatedly asks “Does any of you know whether you have mud on your forehead ?". This time, after the question has been asked k times, the k muddy children will answer ”yes ."

13.Now consider two scenarios: The father repeatedly asks “Does any of you know whether you have mud on your forehead?". All children answer “no " the first time, and continue to answer “no " to repetitions of the same question. The father tells the children that at least one of them is muddy and repeatedly asks “Does any of you know whether you have mud on your forehead ?". This time, after the question has been asked k times, the k muddy children will answer ”yes ."

14.The Modal Logic KT45 n Many applications require models with several interacting agents. We next generalize the logic KT45 by introducing multiple versions of the 2 operator as well as new modal operators. Let A be a set, the elements of which are called agents.

15.The Modal Logic KT45 n Many applications require models with several interacting agents. We next generalize the logic KT45 by introducing multiple versions of the 2 operator as well as new modal operators. Let A be a set, the elements of which are called agents.

16.The Modal Logic KT45 n Many applications require models with several interacting agents. We next generalize the logic KT45 by introducing multiple versions of the 2 operator as well as new modal operators. Let A be a set, the elements of which are called agents.

17.The Modal Logic KT45 n Many applications require models with several interacting agents. We next generalize the logic KT45 by introducing multiple versions of the 2 operator as well as new modal operators. Let A be a set, the elements of which are called agents.

18.(The clauses for the propositional connectives are similar as in the definition of other satisfaction relations.)

19.Formalizing the Wise-Men Puzzle

20.Formalizing the Wise-Men Puzzle

21.Formalizing the Muddy-Children Puzzle

22.Formalizing the Muddy-Children Puzzle