Solution
= Solution
A subset $U$ of a <metric space> is <open> if every $x\in U$ has a radius $r>0$ with the <open ball> $B_d(x,r)\subseteq U$.
= Solution
A subset $U$ of a <metric space> is <open> if every $x\in U$ has a radius $r>0$ with the <open ball> $B_d(x,r)\subseteq U$.