# R(=real Nos)

1. Apr 21, 2009

### poutsos.A

Given that f is a function from R(=real Nos) to R continuous on R AND ,A any subset of R,IS THE closure of f(A) ,a closed set??

2. Apr 22, 2009

### mathman

Re: closure

The closure of any set is by definition a closed set. I think you should rephrase your question.

Last edited: Apr 22, 2009
3. Apr 22, 2009

### poutsos.A

Re: closure

Yes, you right thank you. But if we define a set to be closed if its complement is open,
how then we prove its closure to be a closed set??

Last edited: Apr 22, 2009
4. Apr 23, 2009

### HallsofIvy

Re: closure

What definition of "closure of A" are you using?

5. Apr 23, 2009

### ice109

Re: closure

what? by definition the closure of a set A is the smallest closed set that contains A.

6. Apr 23, 2009

### Werg22

Re: closure

I presume the OP had in the mind the definition that the closure of S is the union of S and the set of its limit points. In this case:

Denote by S' the closure of S. Then we wish to show that S' is closed. Suppose x is in the complement of S'. Then x is not in S and is not a limit point of S. So there is an open ball around x that doesn't intersect S. This open ball cannot contain any limit point of S since if y is inside it, then there is a smaller ball centered at y contained in the bigger - and so there is an open ball around y that doesn't intersect S, so y is not a limit point of S. It follows that the open ball around x does not intersect S'. Therefore the complement of S' is open; so S' is closed.