format updates

content/Math4121/Math4121_L18.md

@@ -12,13 +12,16 @@ By modifying this example, we can find similar with any outer content between 0
 
 $S\subseteq[0,1]$ is perfect if $S=S'$.
 
-Example:
+<details>
+<summary>Examples of perfect set</summary>
 
 - $[0,1]$ is perfect
   - perfect sets are closed
 - Finite collection of points is not perfect because they do not have limit points.
   - perfect sets are uncountable (no countable sets can be perfect)
 
+</details>
+
 #### Middle third Cantor set
 
 We construct the set by removing the middle third of the interval.
@@ -49,7 +52,8 @@ $$
 
 $C$ is perfect and nowhere dense, and outer content is 0.
 
-Proof:
+<details>
+<summary>Proof</summary>
 
 (i) $c_e(C)=0$
 
@@ -70,3 +74,4 @@ It is sufficient to show $C$ contains no intervals.
 Any open intervals has a real number with 1 in it's base 3 decimal expansion (proof in homework)
 
 _take some interval in $(a,b)$ we can change the digits that is small enough and keep the element still in the set_
+</details>