# Indexed Family fo Subsets

1. Sep 3, 2008

### pezola

1. The problem statement, all variables and given/known data

Let f: A $$\rightarrow$$ B be given and let {X$$_{\alpha}$$} for $$\alpha$$ $$\in$$ I be an indexed family of subsets of A.

Prove:

a) f(U$$_{\alpha\inI}$$ X$$_{\alpha}$$) = U$$_{\alpha\inI}$$f(X$$_{\alpha}$$)

3. The attempt at a solution

To prove these two things are equal I must show that the left side is a subset of the right and that the right side is a subset of the left. However, the notation on these problems is really confusing me. I understand that I am being asked to show that the function f applied to the union of all the X$$_{\alpha}$$ s is equal to the union of what you get after you apply the function f to the X$$_{\alpha}$$s. And the result seems reasonable to me, but I have no idea how to right this out.

There are actually many more parts to this question, but I think I will be able to do them once I understand how to write things out.

(I had a hard time getting the symbols to type in right, so I am including a scanned version of the problem as well. Thanks.)

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2. Sep 3, 2008

### tiny-tim

Hi pezola!

(you have to leave a space after \in )

Hint: start "if y ∈ f($$\bigcup_{\alpha\in I}X_{\alpha}$$), then ∃ x ∈ $$\bigcup_{\alpha\in I}X_{\alpha}$$ such that f(x) = y, so ∃ … "

3. Sep 5, 2008

### pezola

It is amazing the difference a space will make.

Thank you so much!...I was able to do all parts of the problem and even presented them in class!