EDB — 0PY

[0PY] Prerequisites:[0P1]↺↻,[0NH]↺↻.

Let \(D(x,r){\stackrel{.}{=}}\{ y\in X : d(x,y)\le r\} \) be the disk, show that it is closed.

Let \(S(x,r){\stackrel{.}{=}}\{ y\in X : d(x,y)= r\} \) be the sphere, show that it is closed.