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.
Let A = {a1, a2, a3, . . . , an} be a nonempty set of n distinct
natural numbers. Prove that there exists a nonempty subset of A for
which the sum of its elements is divisible by n.
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.)
let
G be a simple graph. show that the relation R on the set of
vertices of G such that URV if and only if there is an edge
associated with (u,v) is a symmetric irreflexive relation on
G
Let {Kn : n ∈ N} be a collection of nonempty compact subsets of
R N such that for all n, Kn+1 ⊂ Kn. Show that K = T∞ n=1 Kn is
compact. Can K ever be the empty set?
Determine if there exist a nonempty set S with operation ⋆ on S
and a nonempty set S′ ⊂ S, which is closed with respect to ⋆,
satisfying the following properties.
1) S has identity e with respect to ⋆. ′
2) e ∈/ S .
3) S′ has an identity with respect to ⋆.
Let ? : R^2 → R be given by ?(?, ?) = √︀ |??|.
a. Show ? is continuous at (0, 0).
b. Show ? does not have a directional derivative at (0, 0) along
(1, 1).
c. Is ? differentiable at (0, 0)?