(11) Prove that a union of two countable sets is countable.
(Hint: the same idea used to show that Z is countable might be
useful.) (Don’t forget that countable sets can be finite.)
(12) We saw in class that N × N ∼ N is countable. Prove that A ×
B is is countable for any countable sets A, B. (Hint: If you can
prove that A × B ∼ N × N then you can use what has already...
4: \textbf{Proof} Prove that if $A$ and $B$ are countable sets,
then $A \cup B$ is countable.
5: Use induction and problem 4 to prove that if $A_1, A_2, ...,
A_m$ are each countable sets, then the union $A_1 \cup A_2 \cup ...
\cup A_m$ is countable.
#5 please
Prove that there exist infinitely many positive real numbers
r such that the equation 2x +
3y + 5z = r has no
solution (x,y,z) ∈ Q × Q × Q.
(Hint: Is the set S
= {2x + 3y +
5z : (x,y,z) ∈ Q × Q × Q}
countable?)
Prove the following Theorems:
1. A finite union of compact sets is compact.
2. Any intersection of compact set is compact.
3. A closed subset of a compact set is compact.
4. Every finite set in IRn is compact.