Provide an example
1) A nested sequence of closed, nonempty sets whose intersection
is empty.
2) A set A that is not compact and an open set B such that A ∪ B is
compact.
3) A set A that is not open, but removing one point from A
produces an open set.
4) A set with infinitely many boundary points.
5) A closed set with exactly one boundary point
1)Prove that the intersection of an arbitrary collection of
closed sets is closed.
2)Prove that the union of a finite collection of closed sets is
closed
prove or disppprove.
Suppose A & B are sets.
(1) A function f has an inverse iff f is a bijection.
(2) An injective function f:A->A is surjective.
(3) The composition of bijections is a bijection.
Let S ⊆ R be a nonempty compact set and p ∈ R. Prove that there
exists a point x_0 ∈ S which is “closest” to p. That is, prove that
there exists x0 ∈ S such that |x_0 − p| is minimal.
Prove the following statements!
1. If A and B are sets then
(a) |A ∪ B| = |A| + |B| − |A ∩ B| and
(b) |A × B| = |A||B|.
2. If the function f : A→B is
(a) injective then |A| ≤ |B|.
(b) surjective then |A| ≥ |B|.
3. For each part below, there is a function f : R→R that is
(a) injective and surjective.
(b) injective but not surjective.
(c) surjective but not injective.
(d)...
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.
Suppose that x is real number. Prove that x+1/x =2 if and only
if x=1.
Prove that there does not exist a smallest positive real number.
Is the result still true if we replace ”real number” with
”integer”?
Suppose that x is a real number. Use either proof by
contrapositive or proof by contradiction to show that x3 + 5x = 0
implies that x = 0.