\magnification 2400 \parindent 0pt \parskip 15pt \pageno=11 \hsize 7truein \vsize 9.2truein \def\u{\vskip -10pt} \def\v{\vskip -6pt} 17. Closed Sets and Limit Points Defn: The set $A$ is {\bf closed} iff $X - A$ is open. Thm 17.1: $X$ be a topological space if and only if the following conditions hold: \v (1) $\emptyset$, $X$ are closed. \v (2) Arbitrary intersections of closed sets are closed. \v (3) Finite unions of closed sets are closed. Thm 17.2: Let $Y$ be a subspace of $X$. Then a set $A$ is closed in $Y$ if and only if it equals the intersection of a closed set of $X$ with $Y$. Thm 17.3: Let $Y$ be a subspace of $X$. If $A$ is closed in $Y$ and $Y$ is closed in $X$, then $A$ is closed in $X$. Defn: The {\bf interior} of $A$ = $Int~A$ = $A^0$ = $\cup_{U^open \subset A}U$ Defn: The {\bf closure} of $A$ = $Cl~A = \overline{A} = \cup_{A \subset F^{closed}} F$ Thm 17.4: Let $Y$ be a subspace of $X$, $A \subset Y$. Let $\overline{A}$ denote the closure of $A$ in $X$. Then the closure of $A$ in $Y$ equals $\overline{A} \cap Y$. Defn: $A$ {\bf intersects} $B$ is $A \cap B \not= emptyset$ Thm 17.5: Let $A$ be a subset of the topological space $X$. (a) $x \in \overline{A}$ if and only if ($x \in U^{open}$ implies $U \cap A \not=\emptyset$). (b) $x \in \overline{A}$ if and only if ($x \in B$ where $B$ is a basis element implies $B \cap A \not=\emptyset$). neighborhood Ex Defn: $x \in X$ is a {\bf limit point} of $A$ iff $U \cap A \ \{x\} \not=emptyset$ for every open set $U$. Defn: $A'$ = the set of all limit points of $A$. Thm 17.6: $\overline{A} = A \cup A'$. Cor 17.7: $A$ closed if and only if $A' \subset A$. Defn: $x_n$ converges to a limit $x$ if for every neighborhood $U$ of $x$, there exists a positive integer $N$ such that $n \geq N$ implies $x_n \in U$. Note: limit point of a set is not the same as limit of a sequence. Defn: $X$ is {\bf Hausdorff space} if for all $x_1, x_2 \in X$ such that $x_1 \not= x_2$, there exists neighborhoods $U_1$ and $U_2$ of $x_1$ and $x_2$, respectively such that $U_1 \cap U_2 = \emptyset$. Thm 17.8: Every finite point set in a Hausdorff space $X$ is closed. Defn: $X$ is $T_1$ if every one point set is closed. Thm 17.9: Let $X$ by $T_1$, $A \subset X$. Then $x$ is a limit point of $A$ is and only if every neighborhood of $x$ contains infinitely many points of $A$. Thm 17.10: If $X$ is Hausdorff, then a sequence of points of $X$ convereges to at most one point of $X$. (IFF?) ${1 \over n}$ converges to in the finite complement topology. Thm 17.11: If $X$ has the order topology, then $X$ is Hausdorff. The product of two Hausdorff spaces is Hausdorff. A subspace of a Hausdorff space is Hausdorff. 17. Continuous Functions Defn: $f^{-1}(V) = \{x ~|~ f(x) \in V \}$. Defn: $f: X \rightarrow Y$ is continuous iff for every $V$ open in $Y$, $f^{-1}(V)$ is open in $X$. Lemma: $f$ continuous if and only if for every basis element $B$, $f^{-1}(B)$ is open in $X$. Lemma: $f$ continuous if and only if for every subbasis element $S$, $f^{-1}(S)$ is open in $X$. Thm 18.1: Let $f: X \rightarrow Y$. Then the following are equivalent: (1) $f$ is continuous. (2) For every subset $A$ of $X$, $f(\overline{A}) \subset \overline{f(A)}$. (3) For every closed set $B$ of $Y$, $f^{-1}(B) is closed in X. (4) For each $x \in X$ and each neighborhood $V$ of $f(x)$, there is a neighborhood $U$ of $x$ such that $f(U) \subset V$. Defn: $f: X \rightarrow Y$ is a homeomorphism iff $f$ is a bijection and both $f$ and $f^{-1}$ is continuous. topological property imbedding Thm 18.2 (a.) (Constant function) The constant map $f: X \rightarrow Y$, $f(x) = y_0$ is continuous. (b.) (Inclusion) If $A$ is a subspace of $X$, then the inclusion map $f: A \rightarrow X$, $f(a) = a$ is continuous. (c.) (Composition) If $f: X \rightarrow Y$ and $g: Y \rightarrow Z$ are continuous, then $g \circ f: X \rightarrow Z$ is continuous. (d.) (Restricting the Domain) If $f: X \rightarrow Y$ is continuous and if $A$ is a subspace of $X$, then the restricted function $f|_A: A \rightarrow Y$, $~f|_A(a) = f(a)$ is continuous. (e.) (Restricting or Expanding the Codomain) If $f: X \rightarrow Y$ is continuous and if $Z$ is a subspace of $Y$ containing the image set $f(X)$ or if $Y$ is a subspace of $Z$, then $g: X \rightarrow Z$ is continuous. (f.) (Local formulation of continuity) If $f: X \rightarrow Y$ and $X = \cup U_\alpha$, $U_\alpha$ open where $f|_{U_\alpha} U_\alpha \rightarrow Y$ is continuous, then $f: X \rightarrow Y$ is continuous. Thm 18.3 (The pasting lemma)> Let $X = A \cup B$ where $A$, $B$ are closed in $X$. Let $f: A \rightarrow Y$ and $g: B \rightarrow Y$ be continuous. If $f(x) = g(x)$ for all $x \in A \cap B$, then $h:X \rightarrow Y$, $h(x) = \cases{f(x) & $x \in A$ \cr g(x) & $x \in B$} is continuous. THm 18.4: Let $f: A \rightarrow X \times Y$ be given by the equations $f(a) = (f_1(a), f_2(a))$ where $f_1: A \rightarrow X$, $f_2: A \rightarrow X$. Then $f$ is continuous if and only if $f_1$ and $f_2$ are continuous. 19. The Product Topology. Defn: Let $J$ be an index set. Given a set $X$, a {\bf J-tuple} of elements of $X$ is a function ${\bf x}: J \rightarrow X$. The {\bf $\alpha$th coordinate of x} = $x_\alpha$ = {\bf x}$(\alpha)$ \end Let $\pi_j: \Pi_{i=0}^n X_i \rightarrow X_j$, $\pi_j(x_1, ..., x_n) = x_j$. $\pi_j$ is the projection of $\Pi_{i=0}^n X_i $ onto the $j$th component. Note: