[FOM] Computability question

[Daniel Schwartz]

Dear FOM list,

Would anyone know the answer to the following?

It is well known that the decidable languages are closed with respect to
union and intersection.  What about the undecidable languages?

I'm guessing that these are not closed with respect to either union or
intersection, since I have been unable to find proofs, but I don't know of
any counterexamples.

Any light that anyone can shed on this would be appreciated.

Dan Schwartz

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Dr. Daniel G. Schwartz                            Office    850-644-5875
Dept. of Computer Science, MC 4530                CS Dept   850-644-4029
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