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1. • CommentRowNumber1.
   • CommentAuthornLab edit announcer
   • CommentTimeJun 9th 2022
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   Author: nLab edit announcer
   Format: MarkdownItexPage created, but author did not leave any comments. Anonymous <a href="https://ncatlab.org/nlab/revision/The+real+projective+spaces+in+homotopy+type+theory/1">v1</a>, <a href="https://ncatlab.org/nlab/show/The+real+projective+spaces+in+homotopy+type+theory">current</a>

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   Anonymous

   v1, current

2. • CommentRowNumber2.
   • CommentAuthorUrs
   • CommentTimeJun 9th 2022
   • (edited Jun 9th 2022)
   • PermaLink
   Author: Urs
   Format: MarkdownItexI have made the requested link to *tautological bundle* work: through the existing *[[tautological line bundle]]*. Also added a bunch of further hyperlinks, notably to *[[real projective space]]*. In doing so, I took the liberty of re-formatting as follows. On the off-chance that you are reading the nForum, let me know what you think: *** * [[Ulrik Buchholtz]], [[Egbert Rijke]]: **The real projective spaces in homotopy type theory** Proceedings of the 32nd Annual ACM/IEEE Symposium on Logic in Computer Science LICS 2017 **86** (2017) 1–8 [arxiv:1704.05770](https://arxiv.org/abs/1704.05770) [doi:10.5555/3329995.3330081](https://dl.acm.org/doi/10.5555/3329995.3330081) on the [[real projective space]] in [[homotopy type theory]]. > **Abstract.** [[homotopy type theory|Homotopy type theory]] is a version of [[Martin-Löf dependent type theory|Martin-Löf type theory]] taking advantage of its [[relation between category theory and type theory|homotopical models]]. In particular, we can use and construct [[objects]] of [[homotopy theory]] and reason about them using [[higher inductive types]]. In this article, we construct the [[real projective spaces]], key players in [[homotopy theory]], as certain [[higher inductive types]] in [[homotopy type theory]]. The classical definition of [[real projective space|$\mathbb{R}P^n$]], as the [[quotient space]] identifying [[antipode|antipodal points]] of the [[n-sphere|$n$-sphere]], does not translate directly to homotopy type theory. Instead, we define $\mathbb{R}P^n$ by [[induction]] on $n$ simultaneously with its [[tautological line bundle|tautological bundle]] of $2$-element sets. As the base case, we take $\mathbb{R}P^{-1}$ to be the [[empty type]]. In the [[induction|inductive step]], we take $\mathbb{R}P^{n+1}$ to be the [[mapping cone]] of the projection map of the [[tautological line bundle|tautological bundle]] of $\mathbb{R}P^{n}$, and we use its universal property and the [[univalence axiom]] to define the [[tautological line bundle|tautological bundle]] on $\mathbb{R}P^{n+1}$. By showing that the total space of the [[tautological line bundle|tautological bundle]] of $\mathbb{R}P^n$ is the [[n-sphere]], we retrieve the classical description of $\mathbb{R}P^{n+1}$ as $\mathbb{R}P(n)$ with an $(n+1)$-cell [[space attachment|attached]] to it. The infinite dimensional [[real projective space]], defined as the [[sequential colimit]] of the $\mathbb{R}P^n$ with the canonical inclusion maps, is equivalent to the [[Eilenberg-MacLane space]] $K(\mathbb{Z}/2\mathbb{Z},1)$, which here arises as the subtype of the [[universe]] consisting of $2$-element types. Indeed, the infinite dimensional [[projective space]] classifies the $0$-sphere bundles, which one can think of as synthetic line bundles. These constructions in homotopy type theory further illustrate the utility of homotopy type theory, including the interplay of [[type theory|type theoretic]] and [[homotopy theory|homotopy theoretic]] ideas. *** <a href="https://ncatlab.org/nlab/revision/diff/The+real+projective+spaces+in+homotopy+type+theory/2">diff</a>, <a href="https://ncatlab.org/nlab/revision/The+real+projective+spaces+in+homotopy+type+theory/2">v2</a>, <a href="https://ncatlab.org/nlab/show/The+real+projective+spaces+in+homotopy+type+theory">current</a>

   I have made the requested link to tautological bundle work: through the existing tautological line bundle.

   Also added a bunch of further hyperlinks, notably to real projective space.

   In doing so, I took the liberty of re-formatting as follows. On the off-chance that you are reading the nForum, let me know what you think:

   ---

   • Ulrik Buchholtz, Egbert Rijke:

     The real projective spaces in homotopy type theory

     Proceedings of the 32nd Annual ACM/IEEE Symposium on Logic in Computer Science

     LICS 2017 86 (2017) 1–8

     arxiv:1704.05770

     doi:10.5555/3329995.3330081

   on the real projective space in homotopy type theory.

   Abstract. Homotopy type theory is a version of Martin-Löf type theory taking advantage of its homotopical models. In particular, we can use and construct objects of homotopy theory and reason about them using higher inductive types. In this article, we construct the real projective spaces, key players in homotopy theory, as certain higher inductive types in homotopy type theory. The classical definition of $\mathbb{R}P^n$, as the quotient space identifying antipodal points of the $n$-sphere, does not translate directly to homotopy type theory. Instead, we define $\mathbb{R}P^n$ by induction on $n$ simultaneously with its tautological bundle of $2$-element sets. As the base case, we take $\mathbb{R}P^{-1}$ to be the empty type. In the inductive step, we take $\mathbb{R}P^{n+1}$ to be the mapping cone of the projection map of the tautological bundle of $\mathbb{R}P^{n}$, and we use its universal property and the univalence axiom to define the tautological bundle on $\mathbb{R}P^{n+1}$. By showing that the total space of the tautological bundle of $\mathbb{R}P^n$ is the n-sphere, we retrieve the classical description of $\mathbb{R}P^{n+1}$ as $\mathbb{R}P(n)$ with an $(n+1)$-cell attached to it. The infinite dimensional real projective space, defined as the sequential colimit of the $\mathbb{R}P^n$ with the canonical inclusion maps, is equivalent to the Eilenberg-MacLane space $K(\mathbb{Z}/2\mathbb{Z},1)$, which here arises as the subtype of the universe consisting of $2$-element types. Indeed, the infinite dimensional projective space classifies the $0$-sphere bundles, which one can think of as synthetic line bundles. These constructions in homotopy type theory further illustrate the utility of homotopy type theory, including the interplay of type theoretic and homotopy theoretic ideas.

   ---

   diff, v2, current