Suppose A ≤ B and B ≤ C and C is undecidable, then:
Question
Suppose A ≤ B and B ≤ C and C is undecidable, then:
Solution
If A ≤ B and B ≤ C and C is undecidable, then A is also undecidable.
Here's why:
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A ≤ B means that A is reducible to B. In other words, any instance of problem A can be "translated" into an instance of problem B.
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B ≤ C means that B is reducible to C. Any instance of problem B can be translated into an instance of problem C.
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If C is undecidable, it means there is no algorithm that can determine the answer for all possible inputs of problem C.
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Because A can be reduced to B and B can be reduced to C, any instance of problem A can ultimately be translated into an instance of problem C.
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Therefore, if C is undecidable, then A must also be undecidable, because if we could decide A, we could use that to decide C, which contradicts the assumption that C is undecidable.
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