Let X be an uncountable set, let τf be the finite complement
topology on X, and let τc be the countable complement
topology; namely, we have
τf ={U⊂X : X\U is finite}∪{∅},
τc={U⊂X : X\U is countable}∪{∅},
where “countable” means that the set is either finite or
countably infinite (in bijection with the natural numbers).
(a) What are the compact subspaces of (X, τf )? Are
all compact subspaces closed in (X, τf )?
(b) What are the compact subspaces...
Let f : [0, 1] → R and suppose that, for all finite subsets of
[0, 1], 0 ≤ x1 < x2 < · · · < xn ≤ 1,
we have |f(x1) + f(x2) + · · · + f(xn)| ≤ 1. Let S := {x ∈ [0,
1] : f(x) ̸= 0}. Show that S is countable
Let S be a set of n numbers. Let X be
the set of all subsets of S of size k, and let
Y be the set of all ordered k-tuples
(s1, s2, ,
sk)
such that
s1 < s2
< < sk.
That is,
X
=
{{s1, s2, ,
sk} | si S and all si's
are distinct}, and
Y
=
{(s1, s2, ,
sk) | si S and s1 <
s2 < < sk}.
(a) Define a one-to-one correspondence
f : X → Y.
Explain...
Let X be a set and A a σ-algebra of subsets of X. (a) What does
it mean for a function f : X → R to be measurable? [2%] (b) If f
and g are measurable and α, β ∈ R show that the function αf + βg is
also measurable. [7%] (c) (i) Suppose that f is a measurable
function. Is |f| measurable? (Give a proof or a counterexample.)
[3%] (ii) Suppose that |f| is a measurable function....
Q3 [17% ] Let X be a set and A a σ-algebra of subsets of X.
(a) What does it mean for a function f : X → R to be measurable?
[2%] If f and g are measurable, show that the function f − g is
also measurable. [6%]
(b) Let (fn) be a sequence of measurable functions.
(i) What does it mean to say that (fn) converges pointwise to a
function f? [2%]
(ii) If (fn) converges pointwise...
Let Ω be any set and let F be the collection of all subsets of Ω
that are either countable or have a countable complement. (Recall
that a set is countable if it is either finite or can be placed in
one-to-one correspondence with the natural numbers N = {1, 2, . .
.}.)
(a) Show that F is a σ-algebra.
(b) Show that the set function given by
μ(E)= 0 if E is countable ;
μ(E) = ∞ otherwise...
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...
Let V be a finite dimensional vector space over R. If S is a set
of elements in V such that Span(S) = V ,
what is the relationship between S and the basis of V ?