MHB Is $B - A$ always similar to $B$ if $A$ is countable and $B$ is uncountable?

  • Thread starter Thread starter Dustinsfl
  • Start date Start date
AI Thread Summary
If $A$ is a countable set and $B$ is an uncountable set, it is proven that $B - A$ is similar to $B$. In the first case, when $|A|$ is a finite integer, the cardinality of $B - A$ remains $2^{\aleph_0}$, confirming its equinumerous nature to $B$. In the second case, when $|A|$ is countably infinite, the same conclusion holds as $|B - A|$ also equals $2^{\aleph_0}$. The discussion highlights that removing a countable subset from an uncountable set does not change its cardinality. Thus, $B - A$ is indeed similar to $B$.
Dustinsfl
Messages
2,217
Reaction score
5
If $A$ is a countable set and $B$ an uncountable set, prove that $B - A$ is similar to $B$.Case 1: $|A| = n\in\mathbb{Z}^+$
Since $B$ is uncountable, $|B| = 2^{\aleph_0}$.
Then $|B - A| = 2^{\aleph_0} - n = 2^{\aleph_0}$.
Therefore, $B - A$ is equinumerous to $B$, and hence $B - A$ is similar to $B$.Case 2: $|A| = \aleph_0$
Again, we have $|B - A| = 2^{\aleph_0} - \aleph_0 = 2^{\aleph_0}$
Therefore, $B - A$ is equinumerous to $B$, and hence $B - A$ is similar to $B$.

Does this work?
 
Physics news on Phys.org
What does "similar" mean?

dwsmith said:
Since $B$ is uncountable, $|B| = 2^{\aleph_0}$.
This is wrong. Also, even for finite sets, |B - A| is not necessarily |B| - |A|.
 
dwsmith said:
If $A$ is a countable set and $B$ an uncountable set, prove that $B - A$ is similar to $B$.
There is completely trivial proof if A is a subset of B.
You know that the union of two countable sets is countable.
You also know that $B=A\cup(B-A)$. What if $B-A$ were countable?
 
Last edited:
I was reading documentation about the soundness and completeness of logic formal systems. Consider the following $$\vdash_S \phi$$ where ##S## is the proof-system making part the formal system and ##\phi## is a wff (well formed formula) of the formal language. Note the blank on left of the turnstile symbol ##\vdash_S##, as far as I can tell it actually represents the empty set. So what does it mean ? I guess it actually means ##\phi## is a theorem of the formal system, i.e. there is a...
Back
Top