Let A be a subset of a metric space such that A ⊆ B (p, r) for some p ∈ X and r > 0.
Show that diam(A) ≤ 2r.
B(p,r)=(p-r,p+r)
diam( B(p,r) )=sup{d(a,b)│a,b∈B(p,r) }=d(p-r,p+r)= 2r
Since A ⊆ B (p, r), the diameter of A is less than the diameter of B (p, r):
diam(A)≤2r
Is it true and enough? I...
" Let X be a metric space and p be a point in X and be a positive real number. Use the definition of openness to show that the set U(subset of X) given by:
U = {x∈X|d(x,p)>r} is open. "
I have tried:
U is open if every point of U be an interior point of U. x is an interior point of U if there...
Then the intersection of set of interior points of U (that equals to U, since U is open) and set of interior points of A equals to the set of interior points of the intersection of (U and A). And the set of interior points of (U and cl(A) ) equals to intersection of U and the set of interior...
But I don't know how to prove the opposite direction of this question.
I have tried:
using contradiction, it means the intersection of U and cl(A) is not empty.
I want to prove this theorem:
" Let U be an open subset of a metric space X,and A be an arbitrary subset of X. Prove that the intersection of U and closure of A is empty if and only if the intersection of U and A be empty. "
I've proved one direction of this theorem:
If the intersection of U...
it means that the intersection of U and the closure of A is a set S which contains a point z that is in U and closure of A both. Actually I don't know how to continue the rest of the prove...
I have proved just one direction of this question:
If the intersection of U and closure of A is empty then the intersection of U and A is empty too.
The closure of A is equal to the union of A and the set of all limit points(accumulation points) of A. Then we can use this definition of the...