# Equality of two balls in a metric space

• Sep 28th 2013, 07:23 PM
mrmaaza123
Equality of two balls in a metric space
Is it posssible for b[x:r) and b[y;s) to be equal with x not equal to y and r not equal to s ?
I know it is possible,for instance if we consider a non empty set X with the discrete metric, then for each x in X the balls b[x;r) for r in (0,1] are equal to the singleton set {x}. Also the balls b[x;r) for r in (1,infinity) are equal to X for all x in X.
What is the idea behind two balls with different radii and centre's being equal ?
What i don't understand is, that even in the above example, in what sense are the two balls equal ?
What is the meaning of equality of two balls in a metric space ?
In this example one ball has only singleton element {x} and the other one is the whole metric space X then how are they equal ?

I am a little confused !
• Sep 28th 2013, 07:55 PM
Plato
Re: Equality of two balls in a metric space
Quote:

Originally Posted by mrmaaza123
Is it posssible for b[x:r) and b[y;s) to be equal with x not equal to y and r not equal to s ? I know it is possible,for instance if we consider a non empty set X with the discrete metric, then for each x in X the balls b[x;r) for r in (0,1] are equal to the singleton set {x}. Also the balls b[x;r) for r in (1,infinity) are equal to X for all x in X.
What is the idea behind two balls with different radii and centre's being equal ?

Those of trained in the tradition of R L Moore are distrustful of empty point sets.
One of the most basic properties of metric is: if $\displaystyle x\ne y$ then $\displaystyle d(x,y)>0$.
Now if $\displaystyle x\ne y$ then let $\displaystyle r=\frac{d(x,y)}{2}>0$.

Then it should be very clear that $\displaystyle \mathfrak{B}\left( {x;r} \right) \cap \mathfrak{B}\left( {y;r} \right) = \emptyset$.

How could they be equal ?
• Sep 28th 2013, 08:10 PM
mrmaaza123
Re: Equality of two balls in a metric space
The interval in which r lies is changing so isn't "r" changing ? How can we take it to be the same for both the cases ?
• Sep 29th 2013, 03:49 AM
Plato
Re: Equality of two balls in a metric space
Quote:

Originally Posted by mrmaaza123
The interval in which r lies is changing so isn't "r" changing ? How can we take it to be the same for both the cases ?