- #1
JulienB
- 408
- 12
Homework Statement
Hi everybody! I'd like to check with you guys if I tackled that problem correctly. I might have a few theoretical questions along the way :)
Prove that the difference ##A \setminus B## of a closed set ##A \subset \mathbb{R}^2## and an open set ##B \subset \mathbb{R}^2## is a closed set.
Homework Equations
First I'd like to define open/closed sets in ##\mathbb{R}^2##:
- a set ##M_1 \subset \mathbb{R}^2## is called open, if none of its boundary points is included in the set;
- a set ##M_2 \subset \mathbb{R}^2## is called closed, if it contains all of its boundary points.
I will use also the following theorems:
1. If ##X## is a topological space and ##U## is a subset of ##X##, then the set ##U## is called closed when its complement ##X \setminus U## is an open set.
2. The intersection of two closed sets is a closed set.
The Attempt at a Solution
My first question concerns the topological space. I have read numerous articles (mostly Wikipedia I admit, but not only) about what a topological space is, and I still don't get what it really is. Neither did I understand any of the few examples I saw... I used the theorem nevertheless, assuming ##\mathbb{R}^2## is a topological space, and if not I theorized it may be a subset of ##\mathbb{R}^2## containing both ##A## and ##B##.
Applying the 1st theorem, I can develop as follows:
Let ##X = \mathbb{R}^2## be a topological space (...or not).
##B## is an open set ##\implies## its complement ##B^c = X \setminus B## is closed
##\implies\ A \setminus B = A \cap B^c## is closed, because of theorem 2 (##A## and ##X \setminus B## are closed sets).
Is that a valid proof? Do the general definitions of open/closed sets hold in ##\mathbb{R}^n##?Thanks a lot in advance for your answers, I appreciate your help!Julien.