diff --git a/properties/P000242.md b/properties/P000242.md new file mode 100644 index 000000000..fc08d7924 --- /dev/null +++ b/properties/P000242.md @@ -0,0 +1,37 @@ +--- +uid: P000242 +name: Weakly contractible +aliases: + - Homotopically trivial + - $C^\infty$ +refs: + - wikipedia: Weakly_contractible_space + name: Weakly contractible space on Wikipedia + - zb: "1044.55001" + name: Algebraic Topology (Hatcher) + - zb: "0979.55010" + name: On the universal space for group actions with compact isotropy (Lück, Meintrup) + - zb: "1467.20042" + name: A stronger reformulation of Webb’s conjecture in terms of finite topological spaces (Piterman) + - zb: "0153.52905" + name: Theory of retracts (Borsuk) +--- + +For each integer $n\ge 0$ every continuous map $S^n\to X$ is homotopic to a constant map. + +Equivalently, $X$ is nonempty, {P37} and all [homotopy groups](https://en.wikipedia.org/wiki/Homotopy_group) $\pi_n(X)$ with $n\ge 1$ are trivial. +In other words, $X$ is [weakly homotopy equivalent](https://en.wikipedia.org/wiki/Weak_equivalence_(homotopy_theory)) to {S162}. + +Notation above: $S^n$ is the unit sphere in $\mathbb R^{n+1}$. + +Defined as *weakly contractible* for example on page 2 in {{zb:0979.55010}} +(accessible [here](https://www.academia.edu/53860578/On_the_universal_space_for_group_actions_with_compact_isotropy)). +Defined as *homotopically trivial* on page 281 of {{zb:1467.20042}}. +Borsuk uses the terminology $C^\infty$, defined as $X$ being *$n$-connected* for all $n\ge 0$ +(see page 30 of {{zb:0153.52905}}). + +---- +#### Meta-properties + +- This property is preserved by finite products (see Proposition 4.2 in {{zb:1044.55001}}). +- This property is preserved by homotopy equivalences. diff --git a/theorems/T000890.md b/theorems/T000890.md new file mode 100644 index 000000000..c28f0b844 --- /dev/null +++ b/theorems/T000890.md @@ -0,0 +1,9 @@ +--- +uid: T000890 +if: + P000242: true +then: + P000037: true +--- + +By definition. diff --git a/theorems/T000891.md b/theorems/T000891.md new file mode 100644 index 000000000..5835cf2a2 --- /dev/null +++ b/theorems/T000891.md @@ -0,0 +1,9 @@ +--- +uid: T000891 +if: + P000242: true +then: + P000137: false +--- + +By definition. diff --git a/theorems/T000892.md b/theorems/T000892.md new file mode 100644 index 000000000..2789d708e --- /dev/null +++ b/theorems/T000892.md @@ -0,0 +1,9 @@ +--- +uid: T000892 +if: + P000199: true +then: + P000242: true +--- + +Follows since any homotopy equivalence is also a weak homotopy equivalence. diff --git a/theorems/T000893.md b/theorems/T000893.md new file mode 100644 index 000000000..d19c30ab3 --- /dev/null +++ b/theorems/T000893.md @@ -0,0 +1,9 @@ +--- +uid: T000893 +if: + P000242: true +then: + P000200: true +--- + +By definition, since the first homotopy group is the fundamental group.