- #1
- 858
- 914
Homework Statement
Second try at the proof, this time with correct vocabulary, I hope
Prove that an open ball is an open set.
Homework Equations
The Attempt at a Solution
Let [itex]B(P_0, r)[/itex] be an open ball in [itex]\mathbb{R}^m[/itex], where [itex]P_0[/itex] is the centerpoint of the ball and [itex]r > 0[/itex] is its radius.
Assume point [itex]P\in B(P_0,r)[/itex]. Our objective is to show that every point [itex]P[/itex] in the open ball is an interior point [itex]\Leftrightarrow[/itex] The open ball is an open set.
Therefore [itex]\exists\varepsilon > 0\colon B(P,\varepsilon)\subset B(P_0, r)[/itex]. Assume also point [itex]S\in B(P,\varepsilon)[/itex].
Per the triangle inequality we know: [itex]\forall S\in B(P,\varepsilon) \Rightarrow d(S,P_0)\leq d(S,P) + d(P,P_0)<\varepsilon + d(P,P_0)[/itex].
Fix [itex]\varepsilon\colon = r - d(P,P_0) > 0[/itex] then [itex]\forall S\in B(P,\varepsilon)[/itex];
[itex]d(S,P_0) < \varepsilon + d(P,P_0) = r[/itex], therefore every point in the open ball is an interior point and the open ball is an open set.[itex]_{\blacksquare}[/itex]
Last edited: