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## example :concatenation of 2 undecidable language gives a decidable language [closed]

give example of 2 languages A and B such that A and B are undecidable but there concatenation A.B is decidable.

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This looks like homework to me; I'll vote to close. – Andreas Blass Apr 30 2012 at 0:30
Should I delete my answer? – Joel David Hamkins Apr 30 2012 at 0:58
yes you can delete – shreya Apr 30 2012 at 1:38

## closed as too localized by Andreas Blass, Andres Caicedo, Mark Sapir, Henry Cohn, Benjamin SteinbergApr 30 2012 at 2:33

Let $A$ contain all even-length strings, plus an undecidable collection of odd-length strings. Let $B$ contain all odd-length strings, plus the empty string, plus an undecidable collection of even-length strings. So each is undecidable, but the concatenation $AB$ consists of all strings and hence is decidable.