Let $\displaystyle (E,d)$ be the discrete space.

Compute $\displaystyle S(a,r)$ for $\displaystyle r>1,$ and $\displaystyle S(a,r)$ for $\displaystyle r\le1.$

I know that $\displaystyle d(x,y)$ is defined to be $\displaystyle 1$ for $\displaystyle x\ne y$ and $\displaystyle 0$ for $\displaystyle x=y,$ but I don't know exactly how to use that to solve the problem.