If X is any topological space, a subset A ⊆ X is compact (in the
subspace topology) if and only if every cover of A by open subsets
of X has a finite subcover.
Let U be a subset of a vector space V. Show that spanU is the
intersection of all the subspaces of V that contain U. What does
this say if U=∅? Need proof
Let S be a subset of a vector space V . Show that span(S) =
span(span(S)). Show that span(S) is the unique smallest linear
subspace of V containing S as a subset, and that it is the
intersection of all subspaces of V that contain S as a subset.
Topological Question:
Show that a countable product of a metrizable space is
metrizable. (In the product Topology of course)
ie) Show that d is a metric on X that induces the product
topology on X
The goal is to show that a nonempty subset C⊆R is
closed iff there is a continuous function g:R→R such that
C=g−1(0).
1) Show the IF part. (Hint: explain why the inverse image of a
closed set is closed.)
2) Show the ONLY IF part. (Hint: you may cite parts of Exercise
4.3.12 if needed.)
Definition 1 (Topological space). Let X be a set. A collection O
of subsets of X is called a topology on the set X if the following
properties are satisfied:
(1) emptyset ∈ O and X ∈ O.
(2) For all A,B ∈ O, we have A∩B ∈ O (stability under
intersection).
(3) For all index sets I, and for all collections {Ui}i∈I of
elements of O (i.e., Ui ∈ O for all i ∈ I), we have U i∈I...
Proof:
Let S ⊆ V be a subset of a vector space V over F. We have that S
is linearly dependent if and only if there exist vectors v1, v2, .
. . , vn ∈ S such that vi is a linear combination of v1, v2, . . .
, vi−1, vi+1, . . . , vn for some 1 ≤ i ≤ n.