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单词 43BiinvertibleMaps
释义

4.3 Bi-invertible maps


Using the language introduced in §4.2 (http://planetmath.org/42halfadjointequivalences), we can restate the definition proposed in §2.4 (http://planetmath.org/24homotopiesandequivalences) as follows.

Definition 4.3.1.

We say f:AB is bi-invertibleif it has both a left inverseMathworldPlanetmath and a right inverse:

𝖻𝗂𝗂𝗇𝗏(f):𝗅𝗂𝗇𝗏(f)×𝗋𝗂𝗇𝗏(f).

In §2.4 (http://planetmath.org/24homotopiesandequivalences) we proved that 𝗊𝗂𝗇𝗏(f)𝖻𝗂𝗂𝗇𝗏(f) and 𝖻𝗂𝗂𝗇𝗏(f)𝗊𝗂𝗇𝗏(f).What remains is the following.

Theorem 4.3.2.

For any f:AB, the type biinv(f) is a mere proposition.

Proof.

We may suppose f to be bi-invertible and show that 𝖻𝗂𝗂𝗇𝗏(f) is contractible.But since 𝖻𝗂𝗂𝗇𝗏(f)𝗊𝗂𝗇𝗏(f), by Lemma 4.2.9 (http://planetmath.org/42halfadjointequivalences#Thmprelem4) in this case both 𝗅𝗂𝗇𝗏(f) and 𝗋𝗂𝗇𝗏(f) are contractible, and the product of contractible types is contractible.∎

Note that this also fits the proposal made at the beginning of §4.2 (http://planetmath.org/42halfadjointequivalences): we combine g and η into a contractible type and add an additional datum which combines with ϵ into a contractible type.The difference is that instead of adding a higher datum (a 2-dimensional path) to combine with ϵ, we add a lower one (a right inverse that is separate from the left inverse).

Corollary 4.3.3.

For any f:AB we have biinv(f)ishae(f).

Proof.

We have 𝖻𝗂𝗂𝗇𝗏(f)𝗊𝗂𝗇𝗏(f)𝗂𝗌𝗁𝖺𝖾(f) and 𝗂𝗌𝗁𝖺𝖾(f)𝗊𝗂𝗇𝗏(f)𝖻𝗂𝗂𝗇𝗏(f).Since both 𝗂𝗌𝗁𝖺𝖾(f) and 𝖻𝗂𝗂𝗇𝗏(f) are mere propositions, the equivalence follows from Lemma 3.3.3 (http://planetmath.org/33merepropositions#Thmprelem2).∎

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更新时间:2025/5/4 13:25:14