exercise2sol

# exercise2sol - CSE541 Exercise 2 SOLUTIONS QUESTION 1 Give...

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CSE541 Exercise 2 SOLUTIONS QUESTION 1 Give a definition and an example of a default reasoning. Default reasoning is a reasoning in which it is allowed to draw plausible inferences from less-then- conclusive evidence in the absence of information to the contrary. Example : Consider a statement Birds fly . Tweety, we are told, is a bird. From this, and the fact that birds fly, we conclude that Tweety can fly. This conclusion, however is defeasible : Tweety may be an ostrich, a penguin, a bird with a broken wing, or a bird whose feet have been set in concrete. But as long as we don’t have the evidence to the contrary ( Tweedy has a broken wing ) we accept the conclusion that Tweety can fly . QUESTION 2 Write the following natural language statement: From the fact that it is not necessary that an elephant is not a bird we deduce that: it is not possible that an elephant is a bird or, if it is possible that an elephant is a bird, then it is not necessary that a bird flies. as a formula (i) A 1 ∈ F 1 of a language L , C , I , , , ⇒} , and as a formula (ii) A 2 ∈ F 2 of a language L , , , ⇒} . Solution (i) We translate our statement into a formula A 1 ∈ F 1 of a language L , C , I , , , ⇒} as follows. Propositional Variables: a, b .

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