diff --git a/inputs/lecture_03.tex b/inputs/lecture_03.tex index fb71262..8229f8b 100644 --- a/inputs/lecture_03.tex +++ b/inputs/lecture_03.tex @@ -78,24 +78,24 @@ all condensation points are accumulation points. $P \neq \emptyset$: $\checkmark$ $P \subseteq P'$ (i.e. $P$ is closed): - \begin{IEEEeqnarray*}{rCl} - P &=& \{x \in A | \text{every open neighbourhood of $x$ is uncountable}\}\\ - &\subseteq & \{x \in A | \text{every open neighbourhood of $x$ is at least countable}\} = P'. - \end{IEEEeqnarray*} + % \begin{IEEEeqnarray*}{rCl} + % P &=& \{x \in A | \text{every open neighbourhood of $x$ is uncountable}\}\\ + % &\subseteq & \{x \in A | \text{every open neighbourhood of $x$ is at least countable}\} = P'. + % \end{IEEEeqnarray*} - % Let $x \in P$. - % Let $a < x < b$. - % We need to show that there is some $y \in (a,b) \cap P \setminus \{x\}$. - % Suppose that for all $y \in (a,b) \setminus \{x\}$ - % there is some $a_y < y < b_y$ - % with $(a_y, b_y) \cap A$ being at most countable. - % Wlog.~$a_y, b_y \in \Q$. - % Then - % \[ - % (a,b) \cap A = \{x \} \cup \bigcup_{\substack{y \in (a,b)\\y \neq x}} [(a_y, b_y) \cap A]. - % \] - % But then $(a,b) \cap A$ is at most countable - % contradicting $ x \in P$. + Let $x \in P$. + Let $a < x < b$. + We need to show that there is some $y \in (a,b) \cap P \setminus \{x\}$. + Suppose that for all $y \in (a,b) \setminus \{x\}$ + there is some $a_y < y < b_y$ + with $(a_y, b_y) \cap A$ being at most countable. + Wlog.~$a_y, b_y \in \Q$. + Then + \[ + (a,b) \cap A = \{x \} \cup \bigcup_{\substack{y \in (a,b)\\y \neq x}} [(a_y, b_y) \cap A]. + \] + But then $(a,b) \cap A$ is at most countable + contradicting $ x \in P$. $P' \subseteq P$ : Let $x \in P'$.