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What can you say about union of two non measurable set. They are measurable or not? Is it necessarily true?Thinking about α-cantor set I wonder if the complement of a non measurable set is a non measurable set that does the job.

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If you believe that a non-measurable subset $A$ of $[0,1]$ exists then $A':=[0,1]\setminus A$ will be nonmeasurable as well, but the union $A\cup A'=[0,1]$ is measurable.