# Thread: Proving that AUB is countable Infinite given a specific case

1. ## Proving that AUB is countable Infinite given a specific case

(a) Prove: If A is a ﬁnite set and B is a countable inﬁnite set and A (intersection) B = null set, then AUB is
countable inﬁnite.
(b) Is 'A (intersection) B = null set ' a necessary assumption?

For A, I understand that I need to find a bijection f:B to Natural numbers, then prove that it's 1-1 and onto by proving its invertiblity.

However, where I get confused is how to incorporate that A is a finite set. After proving that B is countable infinite and that it has a bijection with N, then is there a way to prove that if A is finite, then AUB is also countably infinite?

Thank you.

2. ## Re: Proving that AUB is countable Infinite given a specific case

Originally Posted by nexttime35
(a) Prove: If A is a ﬁnite set and B is a countable inﬁnite set and A (intersection) B = null set, then AUB is countable inﬁnite.
(b) Is 'A (intersection) B = null set ' a necessary assumption?
I am sure that you understand that any proof depends upon the definitions in use.
The definitions of finite do differ.

But in any case, if $A$ is finite the $\exists k\in\mathbb{N}$ such that $A\sim\{0,\cdots,k\}$

If $A\cap B=\emptyset$ them just shift the map by $k$.

If $A\cap B\ne\emptyset$ them just shift the map by $k-|A\cap B|$.

### suppose a and b are countably infinite and show aub is countably infinite

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