If \((E,d)\)is metric space and \(x_0\in E\), then the open ball centred at \(x_0\) of radius \(r>0\) is given by the set
—>> \(B(x_0;r)={y\in E:d((x_0,y)<r}\)
\(B(x_0;r)={y\in E:d((x_0,y)\leq r}\)
\(S(x_0;r)={y\in E:d((x_0,y)\geq r}\)
\(S(x_0;r)={y\in E:d((x_0,y)=r}\)
