From c57360cf062805c86fed5d1f3f4adbf52c05f9ff Mon Sep 17 00:00:00 2001
From: Simon-Kor <52245124+Simon-Kor@users.noreply.github.com>
Date: Tue, 3 Sep 2024 05:45:37 +0200
Subject: working commit

---
 library/topology/urysohn2.tex | 44 +++++++++++++++++++++++++++++++------------
 1 file changed, 32 insertions(+), 12 deletions(-)

(limited to 'library/topology/urysohn2.tex')

diff --git a/library/topology/urysohn2.tex b/library/topology/urysohn2.tex
index 9991d21..dbcfc53 100644
--- a/library/topology/urysohn2.tex
+++ b/library/topology/urysohn2.tex
@@ -120,6 +120,18 @@
     Then $X$ is a sequence.
 \end{proposition}
 
+\begin{definition}\label{higher_urysohn_chain}
+    $U$ is a lifted urysohnchain of $X$ iff $U$ is a sequence and $\dom{U} = \naturals$ and for all $n \in \dom{U}$ we have $\at{U}{n}$ is a urysohnchain of $X$ and $\at{U}{\suc{n}}$ is a minimal finer extention of $\at{U}{n}$ in $X$.
+\end{definition}
+
+\begin{definition}\label{staircase}
+    $f$ is a staircase function adapted to $U$ in $X$ iff $U$ is a urysohnchain of $X$ and for all $x,n,m,k$ such that $k = \max{\dom{U}}$ and $n,m \in \dom{U}$ and $n$ follows $m$ in $\dom{U}$ and $x \in (\at{U}{m} \setminus \at{U}{n})$ we have $f(x)= \rfrac{m}{k}$.
+\end{definition}
+
+\begin{definition}\label{staircase_sequence}
+    $f$ is staircase sequence of $U$ iff $f$ is a sequence and $U$ is a lifted urysohnchain of $X$ and $\dom{U} = \dom{f}$ and for all $n \in \dom{U}$ we have $\at{f}{n}$ is a staircase function adapted to $\at{U}{n}$ in $U$.
+\end{definition}
+
 
 
 
@@ -139,6 +151,23 @@
 \end{proof}
 
 
+\begin{theorem}\label{induction_on_urysohnchains}
+    Let $X$ be a urysohn space.
+    Suppose $U_0$ is a sequence.
+    Suppose $U_0$ is a chain of subsets in $X$.
+    Suppose $U_0$ is a urysohnchain of $X$.
+    There exist $U$ such that $U$ is a sequence and $\dom{U} = \naturals$ and $\at{U}{\zero} = U_0$ and for all $n \in \dom{U}$ we have $\at{U}{n}$ is a urysohnchain of $X$ and $\at{U}{\suc{n}}$ is a minimal finer extention of $\at{U}{n}$ in $X$.
+\end{theorem}
+\begin{proof}
+    $U_0$ is a urysohnchain of $X$.
+    It suffices to show that for all $V$ such that $V$ is a urysohnchain of $X$ there exist $V'$ such that $V'$ is a urysohnchain of $X$ and $V'$ is a minimal finer extention of $V$ in $X$.
+    
+\end{proof}
+
+
+
+
+
 \begin{theorem}\label{urysohn}
     Let $X$ be a urysohn space.
     Suppose $A,B \in \closeds{X}$.
@@ -215,20 +244,11 @@
 
     We show that there exist $U$ such that $U$ is a sequence and $\dom{U} = \naturals$ and $\at{U}{\zero} = U_0$ and for all $n \in \dom{U}$ we have $\at{U}{n}$ is a urysohnchain of $X$ and $\at{U}{\suc{n}}$ is a minimal finer extention of $\at{U}{n}$ in $X$.
     \begin{subproof}
-        $U_0$ is a urysohnchain of $X$.
-        We show that if $V$ is a urysohnchain of $X$ then there exist $V'$ such that $V'$ is a urysohnchain of $X$ and $V'$ is a minimal finer extention of $V$ in $X$.
-        \begin{subproof}
-            Omitted.
-        \end{subproof}
-        Let $N' = \naturals$.
-        Let $P = \{C \mid C \in \pow{\pow{X'}} \mid \text{$C$ is a urysohnchain of $X$}\}$.
-        Define $U : N' \to  P$ such that $U(n) =$
-        \begin{cases}
-            &U_0  &\text{if} n = \zero \\
-            &V     & \text{if}  \text{ $n = \suc{m}$ and $V$ is a minimal finer extention of $U(m)$ in $X$}.
-        \end{cases}
+        Follows by \cref{chain_of_subsets,urysohnchain,induction_on_urysohnchains}.
     \end{subproof}
 
+
+
     
 \end{proof}
 
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