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.

Calculus For The Life Sciences
2nd Edition
ISBN:9780321964038
Author:GREENWELL, Raymond N., RITCHEY, Nathan P., Lial, Margaret L.
Publisher:GREENWELL, Raymond N., RITCHEY, Nathan P., Lial, Margaret L.
Chapter12: Probability
Section12.1: Sets
Problem 26E
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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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