Let d is a metric on the set X .
Claim:= The collection of all ϵ -ball Bd(x,ϵ) ,for x∈X and ϵ >0 , is a basis for a topology on X
For each x∈X , x∈B(x,ϵ) for any ϵ >0.
Before checking the second condition for a basis element ,we show that if y is a point of the basis element B(x,ϵ) ,then there is a basis element B(y,δ) centered at y that is contained in B(x,ϵ) .
Define δ=ϵ−d(x,y)>0 as d(x,y)<ϵ.
Then B(y,δ)⊂B(x,ϵ) ,for if z∈B(y,δ) ,then d(y,z)<ϵ−d(x,y), from which we conclude that
d(x,z)≤d(x,y)+d(y,z)<ϵ.
Now to check the second condition for a basis ,let B_1 \ and \ B_2 \
be two basis elements and let y∈B1∩B2 .We have just shown that we can choose positive numbers δ1 and δ2 so that B(y,δ1)⊂B1 and B(y,δ2)⊂B2 . let δ=min(δ1,δ2) .
Thus B(y,δ)⊂B1∩B2 .
As collection of all ϵ -ball B(x,ϵ) form a basis of X .
Thus by definition of basis X induce a Topology.
Comments