diff --git a/properties/P000047.md b/properties/P000047.md index f624caea7..c0ca998e1 100644 --- a/properties/P000047.md +++ b/properties/P000047.md @@ -20,3 +20,4 @@ Some authors (for example {{zb:0684.54001}}) use "totally disconnected" to mean #### Meta-properties - This property is hereditary. +- An arbitrary product of nonempty spaces satisfies this property iff each of its factors does. diff --git a/properties/P000048.md b/properties/P000048.md index 45d313883..84284601e 100644 --- a/properties/P000048.md +++ b/properties/P000048.md @@ -8,11 +8,15 @@ refs: name: Counterexamples in Topology - zb: "0684.54001" name: General Topology (Engelking, 1989) + - mathse: 3642317 + name: The equivalence relation induced by the partition into quasi-components is multiplicative --- Given any two distinct points $x,y \in X$, there exists a clopen subset $A\subseteq X$ such that $x\in A$ and $y\not\in A$. +Equivalently, every quasicomponent of $X$ is a singleton. + Defined on page 32 of {{zb:0386.54001}}. Some authors (for example {{zb:0684.54001}}) use "totally disconnected" to mean {P48} and "hereditarily disconnected" to mean {P47}. @@ -21,3 +25,6 @@ Some authors (for example {{zb:0684.54001}}) use "totally disconnected" to mean #### Meta-properties - This property is hereditary. +- An arbitrary product of nonempty spaces satisfies this property iff each of its factors does.$^{[1]}$ + +[1] use: the quasicomponents are products of quasicomponents in each factor; see {{mathse:3642317}}. diff --git a/properties/P000050.md b/properties/P000050.md index 71f60b222..2f2532755 100644 --- a/properties/P000050.md +++ b/properties/P000050.md @@ -17,5 +17,6 @@ Defined on page 33 of {{zb:0386.54001}}. ---- #### Meta-properties +- $X$ satisfies this property iff its Kolmogorov quotient $\text{Kol}(X)$ does. - This property is hereditary. - This property is preserved by arbitrary products. diff --git a/properties/P000247.md b/properties/P000247.md new file mode 100644 index 000000000..ce20e30dc --- /dev/null +++ b/properties/P000247.md @@ -0,0 +1,27 @@ +--- +uid: P000247 +name: Has connected quasicomponents +refs: + - zb: "0684.54001" + name: General Topology (Engelking, 1989) + - mathse: 3642317 + name: The equivalence relation induced by the partition into quasi-components is multiplicative +--- + +Every quasicomponent of $X$ is connected. +That is, quasicomponents and connected components coincide. + +Equivalently, any two distinct connected components $A$ and $B$ of $X$ can be separated by clopen sets +(i.e., $A\subseteq U$ and $B\subseteq X\setminus U$ for some clopen set $U$). + +The *quasicomponent* of a point $x\in X$ is the intersection of all clopen sets containing $x$. +It is the set of point $y\in X$ that cannot be separated from $x$ by a clopen set. +See definition on p. 356 of {{zb:0684.54001}}. + +---- +#### Meta-properties + +- $X$ satisfies this property iff its Kolmogorov quotient $\text{Kol}(X)$ does. +- An arbitrary product of nonempty spaces satisfies this property iff each of its factors does.$^{[1]}$ + +[1] use: the quasicomponents are products of quasicomponents in each factor; see {{mathse:3642317}}. diff --git a/theorems/T000932.md b/theorems/T000932.md new file mode 100644 index 000000000..d6c6b6d9d --- /dev/null +++ b/theorems/T000932.md @@ -0,0 +1,9 @@ +--- +uid: T000932 +if: + P000234: true +then: + P000247: true +--- + +Every connected component is clopen, hence equal to its quasicomponent. diff --git a/theorems/T000933.md b/theorems/T000933.md new file mode 100644 index 000000000..70fe89d95 --- /dev/null +++ b/theorems/T000933.md @@ -0,0 +1,14 @@ +--- +uid: T000933 +if: + and: + - P000016: true + - P000003: true +then: + P000247: true +refs: + - zb: "0684.54001" + name: General Topology (Engelking, 1989) +--- + +See Theorem 6.23 of {{zb:0684.54001}}. diff --git a/theorems/T000934.md b/theorems/T000934.md new file mode 100644 index 000000000..01a0c681a --- /dev/null +++ b/theorems/T000934.md @@ -0,0 +1,9 @@ +--- +uid: T000934 +if: + P000048: true +then: + P000247: true +--- + +Immediate from the definitions. diff --git a/theorems/T000935.md b/theorems/T000935.md new file mode 100644 index 000000000..d0a1de1aa --- /dev/null +++ b/theorems/T000935.md @@ -0,0 +1,11 @@ +--- +uid: T000935 +if: + P000050: true +then: + P000247: true +--- + +Follows by passing to the Kolmogorov quotient, +since any {P50} {P1} space {P247} +[(Explore)](https://topology.pi-base.org/spaces?q=Zero+dimensional%2BT0%2B%7EHas+connected+quasicomponents). diff --git a/theorems/T000936.md b/theorems/T000936.md new file mode 100644 index 000000000..42609d2e4 --- /dev/null +++ b/theorems/T000936.md @@ -0,0 +1,11 @@ +--- +uid: T000936 +if: + and: + - P000047: true + - P000247: true +then: + P000048: true +--- + +Immediate from the definitions.