Question

In: Advanced Math

prove that if a set A is countably infinite and B is a superset of A,...

prove that if a set A is countably infinite and B is a superset of A, then prove that B is infinite

Expert Solution

Define , for any , where is the set of all natural numbers.

Recall that a set S is called a finite set if there exists , be such there is a bijection between S and and a set is called an infinite set if it is not finite.

Also recall if denotes the cardinality of a set then, , for all n. and for any two sets M and N if there exists an injective map , the we will have .

A set is called countable infinite if there is a bijection between the set and .
Given that , where is countable infinite. To show B is an infinite set.

Proof by contradiction method. Let's assume by contrary that B is a finite set. Then by definition we can find , such that B is bijection with Since , there is an injective map from A to B, Thus we get an injective map from A to Let f be the bijectiion between A and , then , is also a bijective, and A to , is an injective, thus by composing we get an injective map from , thus this means , which is a contradiction. Hence B is an infinite set.

Colby Messinger answered 2 years ago

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