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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closed as too localized by Andreas Blass, Andres Caicedo, Mark Sapir, Henry Cohn, Benjamin Steinberg Apr 30 '12 at 2:33This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, visit the help center. If this question can be reworded to fit the rules in the help center, please edit the question. 


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

