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单词 28TheUnitType
释义

2.8 The unit type


Trivial cases are sometimes important, so we mention briefly the case of the unit type 𝟏.

Theorem 2.8.1.

For any x,y:1, we have (x=y)1.

Proof.

A functionMathworldPlanetmath (x=y)𝟏 is easy to define by sending everything to .Conversely, for any x,y:𝟏 we may assume by inductionMathworldPlanetmath that xy.In this case we have 𝗋𝖾𝖿𝗅:x=y, yielding a constant functionMathworldPlanetmath 𝟏(x=y).

To show that these are inversesPlanetmathPlanetmathPlanetmath, consider first an element u:𝟏.We may assume that u, but this is also the result of the composite 𝟏(x=y)𝟏.

On the other hand, suppose given p:x=y.By path induction, we may assume xy and p is 𝗋𝖾𝖿𝗅x.We may then assume that x is , in which case the composite (x=y)𝟏(x=y) takes p to 𝗋𝖾𝖿𝗅x, i.e. to p.∎

In particular, any two elements of 𝟏 are equal.We leave it to the reader to formulate this equivalence in terms of introduction, elimination, computation, and uniqueness rules.The transport lemma for 𝟏 is simply the transport lemma for constant type families (Lemma 2.3.5 (http://planetmath.org/23typefamiliesarefibrations#Thmlem4)).

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