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I would interpret your statement as "there exists a,b such that z=(a,b)", so both variables are the target of a "there exists", and strictly speaking, that makes them dummy variables, and a reference to a or b in the next sentence doesn't make sense. However, since of the abuse of language I described earlier is so common, I would have interpreted a reference to a or b in the next sentence as if you had made two separate statements: "There exists c,d such that z=(c,d)" and "Let a and b be such that z=(a,b)".reenmachine said:If I were to say that a is in R and b is in [0,1] , would that make sense or would it be irrelevant because a and b are dummy variables?
It makes sense to change "This implies that z is an element of the form (a,b)" to "This implies that z is an element of the form (a,b) with ##a\in\mathbb R## and ##b\in[0,1]##", and I consider this an improvement. But what's really missing from your statement is something that makes it clear that you are assigning values to the variables a and b. (So that it makes sense to refer to them later).
You could e.g. say "define ##a\in\mathbb R## and ##b\in[0,1]## by ##z=(a,b)##" or "let a,b be the unique real numbers such that z=(a,b)"
You said that for all real numbers x,y with y in [0,1], we have z=(x,y). This implies that all of the following equalities and infinitely many more are all true: ##(1,0)=z,\ (-1/12,0)=z,\ (\pi,1/2)=z##...and they all contradict each other.reenmachine said:I'm not sure I understand this , doesn't it say that z=(x,y) and that those x and y are the x in R and y in [0,1] I was talking about?
The last ##\in## should be an equality, but I assume that was just a typo (hmm...repeated at the end of the proof). The statement is fine apart from that. You could say that there's a unique ##x\in\mathbb R## and a unique ##y\in[0,1]## such that z=(x,y) if you want to emphasize that the values of x and y are fully determined by z, but it's not necessary to do that in this proof.reenmachine said:Let ##z \in R × [0,1]## be arbitrary.This implies that there's an ##x \in R## and a ##y \in [0,1]## such that ##z \in (x,y)##.
The "such that" makes the sentence weird. You can end the sentence after ##y\in[0,1]##, and then say "this implies that...". But you don't actually have to mention that ##y\in[0,1]##, since you did that earlier.reenmachine said:This implies that ##x \in \bigcup_{a\in R}\{a\}## and that ##y \in [0,1]## such that ##(x,y) \in \bigcup_{a \in R}\{a\} × [0,1]##.
It looks like what you're trying to do here is what I would say like this: Since ##\mathbb R=\bigcup_{a\in R}\{a\}##, this implies that we have ##z=(x,y)\in\left(\bigcup_{a\in R}\{a\}\right)\times[0,1]##.
But this right-hand side isn't the one we're interested in. We want to prove that ##z\in \bigcup_{a\in R}\left(\{a\}\times[0,1]\right)##.