- #1
faklif
- 18
- 0
I'm using my summer vacation to try to improve my understanding of real analysis on my own but it seems it's not as easy when not having a teacher at hand so I've a small question.
The problem concerns extending a De Morgan relation to more than two sets and then, if possible, to infinity.
From [tex]\neg(A_{1} \cap A_{2}) = \neg A_{1} \cup \neg A_{2}[/tex] I've proved using induction that the same kind of relation holds for n sets [tex]\neg(A_{1} \cap A_{2} \cap ... \cap A_{n}) = \neg A_{1} \cup \neg A_{2} \cup ... \cup \neg A_{n}. [/tex]
So now the problem becomes to explain why induction will not work to extend the relation to an infinite number of sets and then to prove the relation in another way if it is true. I've started on the first part of this but I'm not there yet.
There's a hint that one could possibly look at an earlier problem:
If [tex]A_{1} \supseteq A_{2} \supseteq A_{3} ...[/tex] are all sets containing an infinite number of elements, is the intersection [tex]\cap_{n\in N}A_{n}[/tex] infinite as well?
I think the answer to the previous exercise is no. Looking at [tex]A_{n} = [0,1/n][/tex] which contains an infinite number of elements for any n but when taking the intersection only the element 0 remains. This would meen that there is a fundamental difference between the finite and infinite intersections. I'm not really getting anywhere when it comes to giving a clear reason why induction will not work though.
Homework Statement
The problem concerns extending a De Morgan relation to more than two sets and then, if possible, to infinity.
From [tex]\neg(A_{1} \cap A_{2}) = \neg A_{1} \cup \neg A_{2}[/tex] I've proved using induction that the same kind of relation holds for n sets [tex]\neg(A_{1} \cap A_{2} \cap ... \cap A_{n}) = \neg A_{1} \cup \neg A_{2} \cup ... \cup \neg A_{n}. [/tex]
So now the problem becomes to explain why induction will not work to extend the relation to an infinite number of sets and then to prove the relation in another way if it is true. I've started on the first part of this but I'm not there yet.
Homework Equations
There's a hint that one could possibly look at an earlier problem:
If [tex]A_{1} \supseteq A_{2} \supseteq A_{3} ...[/tex] are all sets containing an infinite number of elements, is the intersection [tex]\cap_{n\in N}A_{n}[/tex] infinite as well?
The Attempt at a Solution
I think the answer to the previous exercise is no. Looking at [tex]A_{n} = [0,1/n][/tex] which contains an infinite number of elements for any n but when taking the intersection only the element 0 remains. This would meen that there is a fundamental difference between the finite and infinite intersections. I'm not really getting anywhere when it comes to giving a clear reason why induction will not work though.
Last edited: