1
$\begingroup$

I found the following lemma in Lee's book on smooth manifolds.

Lemma 1.14 (One-Step Smooth Manifold Structure). Let M be a set, and suppose we are given a collection {Uα} of subsets of M, together with an injective map ϕα : Uα → Rn for each α, such that the following properties are satisfied.
(i) For each α, Ueα = ϕα(Uα) is an open subset of Rn.
(ii) For each α and β, ϕα(Uα ∩ Uβ) and ϕβ(Uα ∩ Uβ) are open in Rn.
(iii) Whenever Uα ∩ Uβ 6= ∅, ϕβ ◦ ϕ−1 α : ϕα(Uα ∩ Uβ) → ϕβ(Uα ∩ Uβ) is smooth.
(iv) Countably many of the sets Uα cover M.
(v) Whenever p, q are distinct points in M, either there exists some Uα containing both p and q or there exist disjoint sets Uα, Uβ with p ∈ Uα and q ∈ Uβ.

Then M has a unique smooth manifold structure such that each (Uα, ϕα) is a smooth chart.

Is condition (v) different than saying that the collection $\{U_{\alpha} \}$ is a covering of $M$?

$\endgroup$
4
  • 1
    $\begingroup$ When you say $\{U_{\alpha}\}$ is a covering of $M$, your $\alpha$ might belong to an uncountable set. Condition $(iv)$ is stronger than that, as it says, $\alpha$ comes from a countable set. $\endgroup$
    – Ajay Kumar Nair
    Mar 1, 2019 at 18:02
  • $\begingroup$ @AjayKumarNair my bad, I wanted to say "condition (v)" not "condition (iv)", I eddited. $\endgroup$
    – roi_saumon
    Mar 1, 2019 at 18:30
  • 1
    $\begingroup$ It is different. There could be $U_{\alpha}, U_{\beta}$, $p\in U_{\alpha},q\in U_{\beta}$, $U_{\alpha}\cap U_{\beta}\neq\phi$. $\endgroup$
    – Teebro Prokash
    Mar 1, 2019 at 19:48
  • $\begingroup$ @TeebroProkash thanks, I get it now! $\endgroup$
    – roi_saumon
    Mar 2, 2019 at 17:38

1 Answer 1

Reset to default
2
$\begingroup$

Condition 5 is there to make sure that the set is given an Hausdorff topology (https://en.m.wikipedia.org/wiki/Hausdorff_space)

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .