Prove that a finite union of countable sets is countable. Therefore, if A is uncountable and B ⊂ A is countable, then A \ B is uncountable.

College Algebra
1st Edition
ISBN:9781938168383
Author:Jay Abramson
Publisher:Jay Abramson
Chapter9: Sequences, Probability And Counting Theory
Section9.5: Counting Principles
Problem 38SE: Suppose a set A has 2,048 subsets. How many distinct objects are contained in A?
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Prove that a finite union of countable sets is countable. Therefore, if A is uncountable and B ⊂ A is countable, then A \ B is uncountable.

 

 

 

 

Expert Solution
Step 1

First of all we have to prove that,

The union of countable family of countable sets is countable.

PROOF:

Without loss of generality, we can denote a countable family of sets by A1, A2,A3

Suppose ai1, ai2, ai3, is an enumeration for Ai. Then,

Yi=1 Ai={ aij : i, j}

To the element aij we assign a natural number n so that there corresponds at most n-1 distinct elements of A.

Therefore, A is countable by Countable Lemma.

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