Thursday, 24 November 2016

real analysis - Does there exist a continuous onto function from mathbbRmathbbQ to mathbbQ?



Does there exist a continuous onto function from RQ to Q?
(where domain is all irrational numbers)



I found many answers for contradicting the fact that there doesnt exist a continuous function which maps rationals to irrationals and vice versa.



But proving that thing was easier since our domain of definition of function was a connected set, we could use that connectedness or we could use the fact that rationals are countable and irrationals are uncountable.




But in this case those properties are not useful. I somehow think that baire category theorem might be useful but I am not good at using it.


Answer



Yes. Say En is the set of irrationals in the interval (n,n+1). Say (qn) is an enumeration of Q. Define f(x)=qn for xEn.


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