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From this statement As there is no surjection from N onto P(N), thus there must exist...

From this statement

As there is no surjection from N onto P(N), thus there must exist an undecidable language.

I would like to understand why similar reasoning does not work with a finite set B which also has no surjection onto P(B)! (with |B|=K and K?N)

Why is there a minimum need for the infinite set?

EDIT Note:

Although I chose an answer, many answers and all comments are important.

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Answer #1

If you take any finite set A of TMs, there is a language not decided by any TM in A and the finite powerset would suffice for that. But this is not what we want. We want to show that there is an undecidable language, i.e. a language that no TM can decide it. The cardinality difference between a finite set and its power set would not show that. You need the cardinality difference with the set of all TMs which is countable to say there is a language which is not decided by any machine in the set.

answered by: ANURANJAN SARSAM
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