96 research outputs found

    Sur les groupes d’homotopie des sphères en théorie des types homotopiques

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    The goal of this thesis is to prove that π4(S3) ≃ Z/2Z in homotopy type theory. In particular it is a constructive and purely homotopy-theoretic proof. We first recall the basic concepts of homotopy type theory, and we prove some well-known results about the homotopy groups of spheres: the computation of the homotopy groups of the circle, the triviality of those of the form πk(Sn) with k < n, and the construction of the Hopf fibration. We then move to more advanced tools. In particular, we define the James construction which allows us to prove the Freudenthal suspension theorem and the fact that there exists a natural number n such that π4(S3) ≃ Z/nZ. Then we study the smash product of spheres, we construct the cohomology ring of a space, and we introduce the Hopf invariant, allowing us to narrow down the n to either 1 or 2. The Hopf invariant also allows us to prove that all the groups of the form π4n−1(S2n) are infinite. Finally we construct the Gysin exact sequence, allowing us to compute the cohomology of CP2 and to prove that π4(S3) ≃ Z/2Z and that more generally πn+1(Sn) ≃ Z/2Z for every n ≥ 3L’objectif de cette thèse est de démontrer que π4(S3) ≃ Z/2Z en théorie des types homotopiques. En particulier, c’est une démonstration constructive et purement homotopique. On commence par rappeler les concepts de base de la théorie des types homotopiques et on démontre quelques résultats bien connus sur les groupes d’homotopie des sphères : le calcul des groupes d’homotopie du cercle, le fait que ceux de la forme πk(Sn) avec k < n sont triviaux et la construction de la fibration de Hopf. On passe ensuite à des outils plus avancés. En particulier, on définit la construction de James, ce qui nous permetde démontrer le théorème de suspension de Freudenthal et le fait qu’il existe un entier naturel n tel que π4(S3) ≃ Z/2Z. On étudie ensuite le produit smash des sphères, on construit l’anneau de cohomologie des espaces et on introduit l’invariant de Hopf, ce qui nous permet de montrer que n est égal soit à 1, soit à 2. L’invariant de Hopf nous permet également de montrer que tous les groupes de la forme π4n−1(S2n) sont infinis. Finalement, on construit la suite exacte de Gysin, ce qui nous permet de calculer la cohomologie de CP2 et de démontrer que π4(S3) ≃ Z/2Z, et que plus généralement on a πn+1(Sn) ≃ Z/2Z pour tout n ≥

    On the classification of certain 1-connected 7-manifolds and related problems

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    In this article, we give a classification of closed, smooth, spin, 1-connected 7-manifolds whose integral cohomology ring is isomorphic to that of CP2×S3\mathbb{C}P^2\times S^3. We also prove that if a closed, smooth, spin, 1-connected 7-manifold has integral cohomology ring isomorphic to that of CP2×S3\mathbb{C}P^2\times S^3 or S2×S5S^2\times S^5, then it admits a Riemannian metric with positive Ricci curvature.Comment: 20 page

    Sphalerons, spectral flow, and anomalies

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    The topology of configuration space may be responsible in part for the existence of sphalerons. Here, sphalerons are defined to be static but unstable finite-energy solutions of the classical field equations. Another manifestation of the nontrivial topology of configuration space is the phenomenon of spectral flow for the eigenvalues of the Dirac Hamiltonian. The spectral flow, in turn, is related to the possible existence of anomalies. In this review, the interconnection of these topics is illustrated for three particular sphalerons of SU(2) Yang-Mills-Higgs theory.Comment: 35 pages with revtex4; invited paper for the August special issue of JMP on "Integrability, topological solitons and beyond
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