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Intersection of Indexed Sets

by gbean
Tags: indexed, intersection, sets
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gbean
#1
Feb22-11, 08:53 PM
P: 43
1. The problem statement, all variables and given/known data
Show that the intersection of Ai (for all i in I = {1, 2, 3, ... n } = A1. Ai is a subset of Aj whenever i <= j.


2. Relevant equations



3. The attempt at a solution
Show:
***I'm having trouble showing part 1***1. that the intersection of Ai is a subset of A1, and
2. A1 is a subset of the intersection of Ai.

This is my attempt: 1. Let x be an element of the intersection of Ai. Then x is in Ai for all i in I. Since A1 is contained in all Ai, then x is contained in A1.

2. Let x be an element of A1, then as A1 is a subset of Aj, for all j >= 1, x is an element of Aj. Thus, x is an element of the intersection of Ai.
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Dick
#2
Feb22-11, 09:07 PM
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I think that's completely correct. Except maybe that you don't need A1 is a subset of Ai for the first part. If x is in the intersection of the Ai, it's certainly in A1.
SammyS
#3
Feb23-11, 12:20 AM
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"This is my attempt: 1. Let x be an element of the intersection of Ai. Then x is in Ai for all i in I. Since A1 is contained in all Ai, then x is contained in A1."

Since x ∈ Ai for all i ∈ I, then clearly, x ∈ A1, because 1 ∈ I .

(Not that what you had was incorrect, but I think this is more direct.)

You could do (2.) by induction.


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